T5C2

Ooh.  That’s not great but it’s enough.  A a 960 will do 1080p@60 no problem.  A 980 will do 2k no problem. 
 
The deal is AMD and Nvidia are set to release new cards very soon.  And unlike the previous 5 years AMD is expected to be really competitive.  As a result this is one of the worst times in the last 5 years to buy a new gpu.  Luckily you don’t have to.  What you got isn’t great, but it’s enough to tide you over.

preamble:  old fart fist bump.  I got a near 20 year old machine in my basement.  I don’t dare plug it into the internet.  
 
If they’re 20 years old they’re good only for testing and maybe not even then.  Might sell well on eBay.
 
1. with ryzen your only choices is am4. What’s a 3650 btw? Do you mean 3950?  I suggest an x570 board for big iron like that and not a cheap one either.  See the motherboard tierlist on this form for what boards can take the draw that thing eats.
 
2. I’m a fan of cheap ass cases.  I’ve got no problems with “nail it to piece of wood” may make my advice less useful.  For a decent case you don’t need to spend more than $60, but it’s hard to spend less because cheaper cases will need fans and the price goes up again.
 
3.  I have an incredibly low opinion of modern rgb.  It’s expensive and proprietary as hell.  Another minority opinion.  See cheap ass cases.
 
4. I’m no help.  I can give you generalitities but no specifics.  Your video card can do 1440p@144hz though, which is effectively 4k even though 1440p is 2k
 
5. the 3950 is a feisty girl and likes a lot of cooling.  Great big air cooler or a 240 or 280 AIO. (I like air better.  It’s more of a pita to install but it lasts much longer and is more reliable)
 
6. there are two things with reading on this forum for that.  Both in the PSU thread..  they’re pinned.  One is the PSU tierlist and he other is titled “how many watts do I need?”
 
7. Singapore. I have heard of shopping wise.  It’s got specific issues and you’ll need advice from another person that lives in Singapore to get anywhere with that.  I don’t.

you mean ryzen 9 3950x. it's probably a bit overkill, but if you have the budget, go for it. i'd recommend the ryzen 9 3900x instead.
i'd go nvidia in a such high end syste. 2080 super, or if you'er building later in the year, they are launching the 3000 series, rumoured to be a significant improvement.
make sure this is 3600C16
samsung ssds are a bit overpriced, but budget doesn't seem to be an issue for you. you should use ssd for software and games and hdd for media files and other storage.
you need a x570 motherboard. I recommend the asus lineup, they are all good. pick based on features, ports and looks.
something with a mesh front for good airflow, like the fractal design meshify C
you can either go for a 280mm or 360mm AIO or for a powerful air cooler like noctua nh-d15 or bequiet dark rock pro 4.
 
look for a 1440p 120-144hz ips monitor - 27in would be ideal
not sure what you mean. case fans?
650W would be enough. look at the tier list in my signature and pick something from tier A.