I built mine for 1200$, without peripherals and monitors. I'm just talking about the tower here.
CPU- YES GET A QUAD CORE they are awesome. I can compile a map 4 times faster than my roommates. Intel makes better quads than AMD.
RAM- grab 2 gigs of DDR2 for great performance, 4 gigs for awesome. I have 4 gigs and I've never slowed down in performance.
Power supply- I recommend a modular one, like the "XION Supernova XON-600F14R-201", which I'm using. You're going to want 600W or higher if you're getting a geforce 8 series.
Mobo- once you've picked all the other parts, fill in the slots on newegg's Advanced Search. Pick a newer model, the type isn't all that important. It just has to fit your case and all your other parts.
Case- pick your favorite color.
I'd also recommend a 8800 gts, because they've got great performance and handle anything Source throws at you, and most other engines as well.