I mostly followed this guide to build my custom gaming PC. Use this benchmark to check which parts are the most bang for your buck.
Motherboard - Asus Z97-A (allows for overclocking)
CPU - Intel Core i5-4690K (3.5GHz) Quad-Core (allows overclocking)
RAM - G.Skill Kit 4GB DDR3 1600MHz Ripjaws CL9 x4 update
GPU - Asus GeForce GTX 970 4GB GDDR5 STRIX (allows overclocking)
PSU - Corsair Modular RM-750W (plenty of power for future updates)
SSD - Crucial MX100 128GB 2.5"
HDD - Seagate Barracuda 1TB 3.5" SATA III 7200rpm x2 update
Cooler - Cooler Master Hyper 212 EVO update
Case - Fractal Design Define R4 Black
I built this 1.5 years ago, and it’s serving me very well, and only updated it once. Perhaps maybe this year or the next I’ll update the CPU.
Anyway, from what research I did, if you’re going for gaming, it’s essential to get a good GPU and RAM, but the CPU is not that important. No need to go around splurging on eight-core or something like that (that’s more important for processing-intense activities like large video editing).
Choose a case that comes with plenty ventilation and dedicated cable organizing so the inside gathers less dust. Obviously the size of the mobo will also influence the size of the case.
You can get away with a 400W PSU, but if you choose a higher power one, it’ll support new updates that require more juice in the future, without changing your PSU in years.
The CPU cooler is not that important unless you do overclocking. If not, the stock cooler is fine.
Put your OS in the SSD for fast boot, and any game you wish to load very fast. The SSDs are pricey, and not many brands are reliable, so choose carefully.
If you have any more questions, I’ll try to answer what I can.