Since you are already using an AMD product I'm guessing you're looking to stay with AMD. You can now find Radon 7970's from MSI for $300. This will give you PLENTY of power to play most of the games out there today. However if you plan on using your machine for projects like Seti@home, then maybe you should look at a NVIDIA card because of the CUDA support for crunching numbers.
with a new GPU and PSU, will i be able to run games with high graphics with the rest of my build? so far my card has been stopping my playability of some high end games like BF3 and GTA 4. I think my processor and ram are fine. thanks !