depends on your processor preference. Some prefer Intel and some prefer AMD. That makes a huge difference. Since I prefer AMD this is the build CPU/mobo/GFX card I would get.
Motherboard - Gigabyte GA A75 UD4H (has four memory slots)
CPU - AMD Phenom II X4 980 Black 3.75 - This is AMD's fastest processor and one tier lower than Intel's fastest processors according to Toms Hiearchy chart.
Vid Card - Gigabyte GV-RG6950 - 1GD Raedon 6950 - This is a one gig card. Newegg has it for $249. I have only seen the 6990 on 4gig setups and the price is insane to be honest. I don't think a personal machine needs one like that but that is just me. In another thread I actual put the same card but from Sapphire but read a review by Toms Hardware that had the Gigabyte rated slightly higher but actually said either would be good.
So for me I would go with both performance and cost in my mindset. The best card I can get for the money. That is why I never even go dual cards. The naked eye would never notice the difference in performance as long as the single card is high quality but dual cards is definatly increased performance though.
FYI.....the Raedon 6950 is the standard vid card that Alienware uses for their best machine. They have the option to upgrade to the 580 though at an increased prices of course.