Below $200, I'd go with AMD .....
......starting with the 560 and up, at 1920 x 1200, however, nVidia is the easy choice, especially in SLI where the factory overclocked versions of the 560 Yi are the same price as the reference versions and faster than the 6970.
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
@ $ 240.00 each, two factory OC'd 6950 Frozr OC in CF get 759 fps at a cost of $ 0.63 per frame
@ $ 320.00 each, two 6970's in CF get 825 fps at a cost of $ 0.78 per frame
@ $ 215.00 each, two factory OC'd 560 Ti's in SLI get 862 fps at a cost of $ 0.50 per frame
That's 100+ fps and $50 cheaper.
At 2560 x 1600, I'd get 6070's