There are important things to decide before you go designing and building a gaming pc, mostly in terms of your expectations:
What kind of games do you intend to play?
Do you want to do 1080, or 4k res? VR?
Do you want to simultaneously do other things with it while gaming, like stream or record?
Do you want good smooth performance with reasonable or medium/high settings, or the highest possible video settings and good performance?
If you are building it yourself, and shooting for nice middle ground visual settings, and reasonable resolution like 1080 in every game, you will want something like a 960/970 geforce card and a good i5 cpu but you won't need to buy the very best ones.
If you have any expectations above that, shoot for an i7 4790k or better, and a higher end 900-1000 series geforce card. Instead of 8 gigs of budget ram, throw in 16gb and get something with higher clock speeds like 1600+mhz from a well known manufacturer like corsair.
If you already have peripherals and monitor, you can expect to build a good i7 powered 1080p rig that runs everything pretty well for around 1000-1500 usd depending on your OS cost, video card and if you used SSDs. For those that can't afford that for a desktop, i5 processors and something like the gtx 770 (or a 960) will suffice for a while, at least until game developers start upping the ante on vid card requirements.