[abhijit_reddevil]

Performance of graphics cards are not only about clock speeds. It also depends on the number of pixel pipelines/vertex per clock. It also depends on the memory interface.

A 7600GT is not anyway better than 7800GT. 7600GT contains 12 pixel pipes, whereas a 7800GT contains 20. Also the former has 128-bit memory interface whereas the latter has 256-bit interface. These are basic reasons why 7800GT is better.