Luddy †

Looks correct to me.
Corsair Type 4 cables. I don't see mention of the gen 3 part on their cable compatibility site, but I'd have to assume that's just an internal revision. Type 4 remains Type 4.

https://www.corsair.com/eu/en/psu-cable-compatibility
The RM550X uses the Type 4 24 pin ATX cable, 
 
The AX Platinum (which the AX760 seems to be) uses a different 24 pin standard ("AX Only").
AX760 and AX860 appear to both be Platinum, meaning they do not use the same 24 pin standard as the Type 4 on the RM550X.
 
So you're looking for a Corsair Type 4 24 pin ATX cable, like @seon123 linked
This is also used by:
- AX1600i
- AX Titanium
- HX Platinum
- RMi
- RM (2019)
- And of course RMX

No
https://www.corsair.com/eu/en/psu-cable-compatibility
 
You have to get any Corsair Type 4 cable. Here is the listing on Corsair's EU website. Click the "Find a Retailer" button to see if it's available in your area. If you're not in Europe, you could still look for the cable, or you could contact Corsair customer support.
https://www.corsair.com/eu/en/Categories/Products/Accessories-|-Parts/PC-Components/Power-Supplies/premium-individually-sleeved-atx-24-pin-cable-type-4-generation-3-config/p/CP-8920158

I think the HyperX ram you chose doesn't work.
You need DDR3 RAM with 1600mhz, not 1866mhz like in your example.
Also the description says it needs CAS latency 11, they HyperX is 10. I'm not sure though, maybe someone can correct me on this.
What should be correct is that you chose a dual kit.
A random recommended kit, even though there are tons of options available, also different depending on locations and DDR3 isn't that common as before:
Crucial CT2K51264BD160B

If you don't care about the higher refreshrate on the Index, then I'd recommend that you go out and find a HTC Vive HMD with basestations and then buy the Index controllers.

Valve index is the "best"
 
otherwise i would highly suggest the oculus Quest (just got mine today) 100% wireless operation for a significantly lower cost than the competition is incredible. and now with Oculus Link, it can be used just like a normal desktop headset flawlessly

The HP Reverb G2 is a VR headset that is coming soon that is made with Steam and Microsoft, and from my understanding the headset is pretty well equipped and is compatible with Valve Index controllers.
https://www8.hp.com/us/en/vr/reverb-g2-vr-headset.html
Here's a quick comparison offered on HP's website to give you an idea about how it stacks up with other VR headsets

Not to mention the EU.

While I understand their reasons, it doesn't seem fair that Apple got a free pass to do all the things Microsoft got into trouble for trying to do.  If they are going to set rules for what you can and cannot build-in at the OS level, it should apply across all OS vendors, not just the biggest at the time.

well it could have been if windows mobile didn't floop like it did. 

Looks like there are already two SSD sleds/mounts on the side, OP just didn't use them  
 
Hopefully the base is heavy enough that whenever you are mounting something on the motherboard, like a GPU, it doesn't start tipping back.

maybe something to mount the ssd on?

So when I started this a couple weeks ago I didn’t have in mind to build a wooden test bench, but since I didn’t have the tools to weld pieces apart from the case, I decided to build a wooden bench with some inspiration from YouTube. I’m quite happy about how it turned out in the end and it’s functionality, though it’s not finished yet as I’m gonna drill holes for the cables to go through and the power button will be drilled to the wood. Just need the tools first
 
 

budget:
PCPartPicker Part List Type Item Price CPU AMD - Ryzen 7 1700 3 GHz 8-Core Processor $159.99 @ Newegg Business Motherboard MSI - B350 TOMAHAWK ATX AM4 Motherboard $94.18 @ OutletPC Memory G.Skill - Aegis 16 GB (2 x 8 GB) DDR4-2800 Memory $69.99 @ Newegg Storage Inland - 480 GB 2.5" Solid State Drive $49.99 @ Amazon Storage Seagate - Barracuda 3 TB 3.5" 7200RPM Internal Hard Drive $74.99 @ Amazon Video Card Gigabyte - GeForce RTX 2060 6 GB WINDFORCE OC Video Card $359.99 @ Amazon Case Phanteks - Eclipse P300 Tempered Glass (Black/White) ATX Mid Tower Case $49.99 @ Newegg Power Supply Cooler Master - MasterWatt 550 W 80+ Bronze Certified Semi-Modular ATX Power Supply $59.95 @ Newegg   Prices include shipping, taxes, rebates, and discounts     Total (before mail-in rebates) $949.07   Mail-in rebates -$30.00   Total $919.07   Generated by PCPartPicker 2019-04-28 11:34 EDT-0400    
Higher budget:
PCPartPicker Part List Type Item Price CPU AMD - Ryzen 7 2700 3.2 GHz 8-Core Processor $223.89 @ OutletPC CPU Cooler ARCTIC - Freezer 34 eSports DUO CPU Cooler $39.99 @ Amazon Motherboard MSI - X470 GAMING PLUS ATX AM4 Motherboard $119.99 @ Newegg Memory Team - Vulcan 16 GB (2 x 8 GB) DDR4-3000 Memory $77.89 @ OutletPC Storage Team - MS30 1 TB M.2-2280 Solid State Drive $103.99 @ Newegg Storage Seagate - Barracuda 2 TB 3.5" 7200RPM Internal Hard Drive $54.99 @ Newegg Storage Seagate - Barracuda 2 TB 3.5" 7200RPM Internal Hard Drive $54.99 @ Newegg Video Card PNY - Quadro RTX 4000 8 GB Video Card $892.99 @ Amazon Case NZXT - H500 (Black) ATX Mid Tower Case $69.17 @ Amazon Power Supply SeaSonic - FOCUS Plus Gold 650 W 80+ Gold Certified Fully-Modular ATX Power Supply $61.98 @ Newegg   Prices include shipping, taxes, rebates, and discounts     Total (before mail-in rebates) $1734.87   Mail-in rebates -$35.00   Total $1699.87   Generated by PCPartPicker 2019-04-28 11:37 EDT-0400   higherer budget:
PCPartPicker Part List Type Item Price CPU AMD - Threadripper 1950X 3.4 GHz 16-Core Processor $519.99 @ Newegg CPU Cooler Noctua - NH-U14S TR4-SP3 82.52 CFM CPU Cooler $79.90 @ Amazon Motherboard ASRock - X399 Taichi ATX TR4 Motherboard $304.98 @ Newegg Memory Corsair - Vengeance LPX 32 GB (4 x 8 GB) DDR4-3000 Memory $169.99 @ Newegg Storage HP - EX920 1 TB M.2-2280 Solid State Drive $134.89 @ OutletPC Storage HP - EX920 1 TB M.2-2280 Solid State Drive $134.89 @ OutletPC Storage Toshiba - X300 5 TB 3.5" 7200RPM Internal Hard Drive $115.89 @ OutletPC Storage Toshiba - X300 5 TB 3.5" 7200RPM Internal Hard Drive $115.89 @ OutletPC Case Fractal Design - Define C TG ATX Mid Tower Case $88.38 @ Amazon Power Supply SeaSonic - FOCUS Plus Platinum 750 W 80+ Platinum Certified Fully-Modular ATX Power Supply $119.86 @ Amazon Other PNY QUADRO RTX 6000 Graphic Card - 16 GB GDDR6 - Dual Slot Space Required - 4096 bit Bus Width - Fan Cooler - OpenACC, OpenGL 4.5, DirectX 12, OpenCL, DirectCompute 5.0, Vulkan 1.0-4 x DisplayPort - $2220.00 @ Amazon   Prices include shipping, taxes, rebates, and discounts     Total (before mail-in rebates) $4064.66   Mail-in rebates -$60.00   Total $4004.66   Generated by PCPartPicker 2019-04-28 11:40 EDT-0400    

heyyyy good luck

Hello,
 
I deal with audio a lot due to my hobbies and where I work.
 
To directly answer your question, you should get an external DAC (Digital Audio Converter) and headphone amp. This will reduce electromagnetic interference coming from your PC, which internal sound cards that have dedicated headphone amps can suffer from.
 
now to supply with more information,

DAC's and amps can come as separate units or in a sort of all-in-one unit. Either would be fine for your situation, you just have to find one that suits your needs. I will not recommend any right now, as I do not know what kind of budget you have, nor do I know your needs outside of it being able to run your headphones.
 
I hope you find this information helpful

Unless you're a hardcore audiophile, you'll never notice the difference...

There is no such thing as "gaming sound". Get an external one. Those don't have to use gaming marketing to sell their stuff.

i really like my Soundblaster Z, but generally the external ones are considered better

Sound is sound. You either like how it sounds or you don't. 
Sound card is usually AMP, you get it if you can't get the volume high enough. 
You get DAC if your audio suffers from noise. 
 
Any modern PC should be capable of having both good enough. 
Unless you're hard-core into audio, don't bother. 

I'd say 99% of the time, don't get a soundcard. As others have stated, external DACs and AMPs are the way to go, but at this point it's just like building a PC. You can "game just fine" on a $800 PC, but some people are enthusiasts and would rather get the best or the best or really customize the look of the system.
 
When it comes to audio, the onboard audio is fine but you can shell out some extra cash for higher quality trinkets.

Most motherboards $100 USD and higher already have pretty good onboard audio, making a soundcard not necessary, unless you're either a pro gamer, or a total audiophile. 

Gaming audio? Not really needed, unless you want a theatrical experience.
For listening to music (and by music I do not mean YouTube, I mean *.flac files etc.), soundcards are definitely better than on-board audio.
But if you were serious about sound you would probably want to invest in DAC/AMPs.

Sound cards have always been dead from a quality audio perspective. Putting sensitive audio electrical components inside an PC with a ton of EMI is a recipe  for disaster. External DAC/Amps however are still very much a thing and used for many headphones in the audiophile space.
 
Also BTW if you are thinking of buying DT990s don't. DT880 are far better.

At 4.6GHz those temps are a little high, especially for that voltage. I also highly recommend de-lidding your CPU if you aim higher.

That sounds like a red lie to me, unless he means he's reached that temperature with his PC off?