Would you mind if I contributed a bit?
I have a GTX 970, runs great. I don't think an Intel i7 is necessary, just get an intel i5 4690k. I have the previous version of it and it still runs great.
I've been hearing from friends on other servers that their Nivida cards have had issues lately in terms of their cards repeatedly being ejected.
As for i5, I use it and it's pretty decent enough.
- Operating System: Windows 7 is free via piracy. No idea about Windows 10 yet. You don't want Windows 8. You don't want Mac because you want to game. Linux is free, and now that Steam is on Linux it's getting better, but not great yet. Suggestion is Win7 (or 10), and if you have am existing copy you might be able to just use it.
From what I've read, Windows 10 is a free upgrade from any Windows 7/8/8.1, even if you've got a pirated copy. I've never used Windows 10 though, so I can't give an opinion. Mac I can't be bothered giving an opinion about, as I know I'll get shot down and this isn't my thread to start a rant. Linux is nice though. Simple, fast, highly adaptable. Bit more work than Windows, but it's a UNIX system which is nice. Noket can give a better opinion if you're serious. Like Ako said though, it's great, but not really up to par with Windows for a main computer (Unless you're Noket, or hate Windows)
Windows 10 is definitely worth upgrading to from 7 or 8(.1). It's smooth, clean, and is far less buggy.
A really good one built from scratch I've seen before consists of the following:
Case: Antec P182
Mobo: Asus P8Z68-V PRO/GEN3 ATX LGA1155
CPU: Intel Core i7-2600K, overclocked to 4.2GHz (I think i5, again, does the job decent enough)
CPU H/S: COOLER MASTER Hyper 212 Plus (You can find these for $20, but I'm pretty sure there are cheaper alternatives)
RAM: CORSAIR Vengeance 16GB (4x 4GB) DDR3 1600
GPU: EVGA SuperClocked GeForce GTX 570 HD
PSU: CORSAIR Professional Series HX750 750W
SSD: Crucial M4 64GB & Samsung 850 EVO 500GB
HDD: 10+ TB of hard disk space, internal and networked backup
OS: Windows 7 Ultimate (but again, you'll prefer Win10 instead)
I have no idea how much that actually costs, but I know for a fact that it's quite decent.
I also have the feeling I'm about to get a lot of hate directed towards me for no apparent reason.