Actually when it comes to the 7600gt and 7900gt, the jump in power from a good overclock is sick.
Look at these benches
http://sg.vr-zone.com/?i=3437&s=451% faster fps!
Sure you dont need the special heatsink - if you have a spare old cpu sink, and are crafty you can latch it down
I don't think the 7600gt is as impressive, but still - It takes a good deal of power to churn out 7500 points in 3dmark, over the standard 6k for it.