The 7600GS is a little slower than 6800GS, assuming you get either the Evga or the XFX XXX edition, which are clocked at 550/850 and 500/900 respectively. 7600GS' clocked at the standard 400/800 will be a lot slower than the 6800GS.
The 6800GS clocks in at 425/1000 standard, and has 256-bit mem compared to the 128-bit on the 7600GS, meaning it'll hold up to AA/AF and higher resolutions a lot better than the 7600GS will.
All the technical crap aside, the 6800GS is faster in all cases, but it goes for $75+ more than the 7600GS does. You can see a comparison
here, but the 6 series is being phased out. The 7600GT is the successor to the 6800GS, it's faster, more readily available, and about the same price.
If you don't mind running in 12x10 or 10x7 with no AA/AF the 7600GS is a hell of a value at ~$100. But once you start adding AA/AF into the mix and getting into 16x12 and higher, the 7600gs' low bandwidth starts to cripple it, and even though the 7600GT costs 50% more, there are cases when it's 50% faster than the 7600GS. And that gap will only increase as more graphically intensive games are released, so the 7600GT is more future proof. Go with the GS if you're on a tight budget and don't care for maximal eye candy, otherwise go with the GT.