Well if you want a high end gaming rig, Reminez is right... the emphasis will be on the graphics card. But you also dont want to forget about having a good amount of memory (RAM) and a beastly CPU. Here are a few rules to go buy when looking for desktop parts for gaming:
CPU:
try to get something at least clocked at 3 GHZ and a multiple core processor for sure. I myself have a AMD phenom 2 955 clocked at 3.2 ghz and that things awesome for running games. Its a beastly quad core. Idk much about Intel processors but I guess the i7 is where its at for gaming. I would do some research into amd and intel and see what suits you best.
Memory:
DDR 3, low latency, and a decent clock (1600 mhz) at least 4 GB
Graphics Card:
This is where it gets kinda messy, finding a good card is hard but you said "perfect" gaming machine so try to get something that has at least 1GB of memory GDDR5 preferably, PCI express 2.0 and at the very least 800 processor cores.
You also want to make sure you have a good motherboard that supports all your parts and a good hard drive (1TB?) to store all your game files on. Games these days are huge! Hope this helps a bit.