Tore Alkeks

850w if you have those gpus and a mainstream cpu so anything on X370,X470,Z370,Z390 but if you have HEDT (high end desktop) or more like X299,X399 then you might want to step it up. what cpu do you have. or just punch in you specs in pcpartpicker.com and see what it thinks you should then add 50w to its recommendation 
 

I'd go for the upgrade too but it'll probably be just about enough to run stock, assuming it is a good quality PSU and depending on the rest of the system. I used to run 2x980Ti and a 6700k off a 650W, with not a lot of headroom...

Sorry took a sec to get back to you, all you need is a basic mixer and 3x3.5mm to RCA cables:
https://www.amazon.com/Rolls-MX42-Stereo-Mini-Mixer/dp/B00102ZN40?crid=R8Z4RAGGM8WW&keywords=stereo+mixer&qid=1540830918&s=Musical+Instruments&sprefix=stereo+mixer%2Cmi%2C186&sr=1-7&ref=sr_1_7
https://www.amazon.com/UGREEN-3-5mm-Auxiliary-Stereo-Splitter/dp/B00LM2Y2U4?keywords=3.5mm+to+rca&qid=1540831039&s=Musical+Instruments&sr=1-7&ref=sr_1_7

Your on-board audio into one input, the display audio into another input and the output to the speakers.

If the 3.5mm cable attached to your speakers isn't modular you will need female 3.5mm to RCA:
https://www.amazon.com/UGREEN-Adapter-Smartphones-Tablets-Theater/dp/B00QUWDUMC?keywords=female+3.5mm+to+rca&qid=1540831276&s=Electronics&sr=1-6&ref=sr_1_6

If you want to keep your old computer with win10 installed, then it's against policy to use the same copy on second computer. It may work, of course, but it doesn't mean it's legal. And it doesn't mean that it will work forever - probably until next time Windows check license.

When it's tied to your microsoft account, you can install a new PC just fine. When it gives an activation error, just say you changed hardware and it should fix itself.

yes you can.  no need new key, just reuse the account log in - setting >activation....
once linked to new pc, you can even reinstall win 10 again so no login prompt needed (then I forgot my ms account lol)