If you wanna go higher, the GTX470 is a great card, though it runs hot! Plus, you'll need a higher power unit for that (650W)
For your resolution, I'd strongly suggest the GTX460 1gb. If you're looking at going the HD route then the 5770. Both will more than serve your purpose.
EDIT: Why I would recommend the GTX460 over the 5770 is coz the 460 sits on a 256-bit bus while the 5770 is on a 128-bit bus.
At that resolution I would suggest a 5770 or a 450, as both cards are under 150$ now and are able to play games on high with at least 2xAA(Possible 8xAA for games before 2008) with only a single 6pin need.
How many 6pin connectors does it have?