Keep in mind that in two years from now you'll get a new graphics card(s) in order to play games based on unreal engine 3.
I doubt 2x6600 can handle the graphics of UE3.
The 6800Ultra is the only one supposed to run the game in "acceptable" framerates and probably at 800x600.
Maybe getting an SLI 6800GT and another one later would be a better solution.
AMD 3500+(NewCastle), Asus A8V Deluxe, 2x512 PC3200
Asus X800XT PE