7900GT = One heck of a lot faster, especially when overclocked with better IQ/features, but in MOST cases will bust your budget @ +/- $270.
EXCEPT: http://www.monarchcomputer.com/Merchant...OD&Product_Code=190593&AFFIL=rtpe&NR=1
7900GS = Considerably faster with better IQ/features & will pretty much nail your budget target @ +/-$200.
7600GT = Similar to slightly faster with better IQ/features & will come in under budget @ +/- $150-$170.
6800GT = Similar to slightly slower with better IQ/features, but not quite as good as the 7 seris cards, pricy @ +/- $200 and getting hard to find.
6800GS = Slightly slower, but with better IQ/features, again not as good as 7 cards, also bit pricy @ +/- $175 & getting harder to find.
I'd go for the 7900GS ... essentially its a cooler running 7800GT with faster clockspeeds.
EDIT: If you can snag that PNY 7900GT, I'd say thats the way to go at the moment!