If you've got a budget of around $130 or so, a HD7770 would do well at that resolution, as would a HD6850. On a tighter budget, look for a HD6770. If your budget is higher, unless you're planning on a monitor upgrade, anything stronger than a HD6780 or GTX560 will likely be overkill.
Confirm that your PSU is sufficient. A quality modern PSU has full range active PFC (no little voltage switch) and some level of 80+ certification. Seasonic, Antec (including the non-active PFC VP-450), Corsair, XFX, and Enermax/LEPA are among the better brands. Any of those cards will run on a quality 500W PSU. Up to the HD6850 or HD7770 will run on a 380W Antec Earthwatts or 400W-430W Corsair CX.