Well, I think you would be CPU limited in most games and you don't exactly have a killer power supply. Based on that and the resolution you have, I would go with a Radeon 6790. It is actually related to the Radeon 6850 and 6870 and not the rest of the 6700 series. It also only requires a single 6 pin connector and will be able to be overclocked without stressing your 520 watt PSU. It can also play most games at 1600x900 with the settings maxed out. Whether or not your CPU is up to the task is another matter completely, but at least the multiplier is unlocked. Just about any nVidia application will use more power.
Lastly, they start at $120 for a basic one. So you can get a really nice one for a little more with a better or more quiet cooling solution and still be in your budget.
Cheers and good luck!