I built my computer and I forget most of the internals. I feel the most important parts of your gaming PC are mouse, keyboard, controller and monitor. The internals just need to be fast enough to run your games, which any halfway-decent computer will be nowadays. It's more important that the stuff you're holding in your hands and staring at all day are high quality.

CPU: i5-3570K that I overclocked a little bit because the BIOS made it really easy
RAM: 16GB because I use this computer for things other than gaming
Hard drive: 1TB of cheap slowness
Mouse: Razer Deathadder, corded because a battery-powered mouse makes me sad
Keyboard: Unicomp mechanical keyboard with state-of-the-art 1980's buckling springs technology
Controller: Razer Onza that I took apart and rebuilt because it had factory defects, would not buy again
Monitor: ASUS VG248, 24" 144hz with 3D so I can run LightBoost, 1ms response time because input lag