Hmm....is changing the PSU an option? I'm not too happy about that brand.
I'd recommend the GTX 460 1gb would be a good call. Here's one for 199 after Mail in rebate:http://www.newegg.com/Product/Product.aspx?Item=N82E16814127510&cm_re=gtx_460_1gb-_-14-127-510-_-Product
Even an HD 5770 would be a good choice for 139:http://www.newegg.com/Product/Product.aspx?Item=N82E16814127476&cm_re=5770-_-14-127-476-_-Product
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.