While the 6770 is a good option, I'm not quite sure the Athlon X2 5200+ will be able to provide enough juice for it without bottlenecking (of course, it depends on the resolution you play at - please mention your monitor and screen resolution as well).
Without knowing the PSU unit, it is difficult to tell. But if I assume your 8600 card works just fine, I guess we can have a look at some cards with roughly similar power draws (note: haven't checked the exact values but I think these cards should be in the same range or just slightly higher than the 8600 GT).
So, on that basis we have the following options:
1) GeForce GT 430
2) Radeon HD 5570
Between these two the 5570 is an easy winner, comparable performance at the minimum, better in most cases, and lower power consumption than the GT 430 (it's almost on par with the 8600 GT, so you shouldn't have problems).
Any one of these cards will probably give you almost twice the performance.
For your CPU, if I remember correctly, the best GPU would be around 8800 GT/GTX levels of performance (otherwise, bottlenecking could occur and you will not be using the GPU fully). So, assuming you do have a good PSU, you can try for a GeForce GT 440 or Radeon HD 5670/6670. Performance wise, the 5670 has a consistent 10% lead over the GT 440.
While you can go for a more expensive and better GPU such as the 6770, it is my personal feeling that perhaps your CPU will limit the performance of any graphics card above the 5670/6670. Therefore, if you want to use higher end graphics cards, a CPU upgrade *may* be recommended.
Prices: HD 5570, GT 430 is available at ~3.5K or less, HD 5670/6670 at ~4.7K or less (GDDR5 version, don't get GDDR3 version), GT 440 at ~4.5K (some 2GB models available at this rate too).
