I have a 6600GT myself.
A friend bought a 7600GS (with DDR3, though - Gainward 7600GS Golden Sample), and I got to test it on my system. It was way better.
As for overclocking:
- my 6600GT only reaches 550MHz/1100MHz with no artifacts (from 500/900).
- the 7600GS went over the 7600GT frequncies. The Gainward card comes with higher frequencies (450/550*2) than the plain 7600GS (400/400*2). I overclocked it stable to 610/1500.