NVIDIA GTX 280 vs 285, and Radeon 4870x2

So I'm building a gaming PC for the first time and I know what I want for everything except in the graphics department.

I've looked at the prices and reviews and the 280 is by far the cheapest at $225. The 1GB 285 is $340, and the 4870x2 is $400. What should I go with? I typically play games like Fallout 3, Crysis, Mass Effect, and STALKER: Clear Sky, and the monitor I'm getting supports up to 1650x1080, although I'm considering getting one which supports a higher res.