I would recommend going with an ATI HD 4870 1GB or 512mb GPU. I've done a lot of research and the 4870 can hang in there with the best of nvidia. Plus the ram is gDDR5 which really doesn't mean too much at the resolution you'll be playing at (same as mine actually) but if you were to ever get a higher resolution monitor, you'd definitely appreciate having that faster ram and a larger quanitity (1GB).
Also, I don't have a problem with a core 216 GTX 260 if you would want to go that route. Personally I don't know too much about the 285 so i'm keeping my mouth shut about it. I'll be looking in to those new AMD Phenoms... Does anyone think the prices on them will drop in the next week or so? Don't know if it'd be worth it to pay about 40-50$ more for .2 GHz over the 940.