If you have never build a computer before, I suggest asking someone (real life) who has, they can help you greatly.
A nice mobo, 2-4 gigs of ram, a graphics card with at least 512 mgs but more is better (also mind its bus speed)
If you suddenly come upon a large sum of money buy the Intel Core i7 Extreme Edition I7-975 Quad Core, its $1000 but worth every penny.
On the cheaper side:
AMD Phenom II X2 555 Black Edition Dual Core: Its $100, its cool, quite, you can unlock 2 more cores and it can be over clocked.
AMD Athlon II X4 620 Propus 2.6GHz 4 core: $100, go with this one if you don't trust yourself to overclock
is a good source for computer hardware reviews/explanations