7600gs is better for watching HD media because it has hardware acceleration. It also supports SM 3.0 found in newer games. However, the 6800GT in terms of gaming performance can easily match or beat the 7600GS and in reality you won't be needing SM 3.0 because its not really that big of a difference than SM 2.0.
Bottom Line: If the 6800GT is cheaper or the same price get it because it will be better at gaming, have a much greater overclocking potential, and faster ram.
Hope it helps