Your gonna need vista64 to run DX10 and 4gigs of RAM!!
$2000 is a pretty large budget.
I say get the top gpu Sapphire ATI Radeon HD 4870 X2 2048MB GDDR5 or,
NVIDIA Geforce 9800 GX2 or if you want to go SLI,
NVIDIA Geforce 8800 Ultra SLI
These all seem to benchmark pretty dam high!
Get 4gigs of RAM, i recommed Corsair.
Processor is a difficult one, either Q6600 or the E8600 3.33GHz which you can easily get 4.0GHz out of.
That leads me into the heatsink: Zalman CNPS9700-NT nVidia Tritium CPU Cooler (Socket 754/939/940/AM2/LGA775)
motherboard: I would get Asus P5Q or something like the Gigabyte GA-EP45-DQ6 (P45) - dont know much about mobos.