Since you already have a good CPU, a decent GPU should do the trick. I still think you should try the agem out on your current setup and see how well it turns out. Here is a list I composed of GPUs depending on budget. It also includes data on recommend wattage for it:
Under $90: HD 6670 (Below 350W)
Under Sub $100: HD 6750 (400W)
Low $100: HD 6770 (400 to 500W)
Mid $100: HD 6850/GTX 460 (400 to 500W)
High $100: HD 6870/GTX 560 (400 to 500W)
Low $200: GTX 560 Ti/HD 6950 1GB (500 to 600W)
Mid $200: GTX 560 Ti/HD 6950 2GB (500W to 600W)
High $200: HD 6950 2GB/GTX 560 Ti 448 Cores (500W to 600W)
L,M,H $300: GTX 570/HD 6970 (500W to 600W)
Anything about $400: GTX 580 (500W to 600W)
probably a little off on the wattage, but nothing to serious.