The 7600GT will own the 6800GS.
Not quite, there similar in performance really. The 6800GS has about a 100mhz slower core clock and a 300mhz memory clock (on average), but it has a 256-bit memory interface where as the 7600GT has those higher clocks but only a 128-bit memory clock. Oddly enough, Nvidia is going the route ATI did with the X1600XT (high clocks, poor memory interface) and ATI is going the route Nvidia did with the 6800GS (slower clocks, better memory interface). No commen sense there at all ... but I'd say that the 7600GT and 6800GS perform nearly identically, but the 7600GT will hold more OCing potential so therefore go with the 7600GT since it's prices will fall rather fast and become cheaper than the typical 6800GS.
Moving along ... there is no reason to buy a 7800 series card
The 7900GT is faster than the 7800GTX yet more than $100 cheaper.
The 7900GT is much faster than the 7800GT and costs the same.
The 7900GTX is faster than the 7800GTX 512 yet nearly 2/3 the price.
What I find interesting is that Nvidia hasn't released a 500/1500 16 or 20 pipeline card with a 256-bit memory interface to fit into around the $250 price range. *cough* 7900GS *cough*
So in conclusion, the only Nvidia cards you want to be looking at are:
7600GT ~$200 (mid ranged gaming)
7900GT ~$300 (high end gaming)
7900GTX ~ $450 (extreme gaming)