Use AMD Athlon 64, it is the best you can get for gaming, forget the clock speed, this one will get you the best performance, you can play the game, online, download music, burn a cd, and listen to music all at the same time and it will not affect you, it will never freeze or crash!
we at the datawhore networks have done some tests on AMD 64 CPUs, and it simply beats Intel!
Since it's a gaming computer, use Microsft Windows XP, even though I don't like it alot!
NVidia makes the best graphic cards!
get at least 512 mb or ram!
get a big harddrive (50 Gbs or more)
if you don't buy the OS, this project should cost you somewhere around $1000 USD.