Well seeing as this is a Gaming computer, you want to focus on the graphics card, i would suggest going with a high end (Not the greatest) ATI card to compliment a core i5 or core i7 processor & Mobo. the rest is really based around that and your personal preferences (Like size of HD).
<ramble>: Be sure you get a big enough power supply, that requires math
. Make sure you get Gigabit (10/100/1000) network card/mobo as anything less is just outdated for a new computer. CD/DVD +- drive (speed not necessarily important). If you know how to over-clock, and plan to, then get Liquid Cooling, if not just make sure you get a heat sink with cooling fan if it doesn't come with Mobo/processor (one of those things i forgot to order and got stuck waiting for).</ramble>
I can estimate all this to be around 550 - 700 (Just a shot in the dark), but that depends on where you buy, do you have a place in mind? Definitely look at different places, i used tiger direct at one point, mixed feelings about that place.
Anyway hope this helped