depends on your standards. id definitely go with the 7800gt over the 7600gt though, im running a 7800gt in a system right now and it runs quake 4 on ultra settings with no lag, and it'll even use 4x AA with barely any lag... i normally leave it at 2x just to smooth it out a bit though.
make sure you've got a pci-express x16 slot available... those cards arent for AGP
btw you can get a 7900gt for $270 or so... that 7800gt seems a little overpriced haha
and looking at your profile... that 7600gt is being sold in the UK... for £107... not $107 lol
whats your budget, and what games do you want to play?