Play allot of games? Then the 6800GT would be nice but if you don't play many games and don't plan too then consider the 6600GT as it's still a great card and wont break the bank.
Edit:
Before you make a decission bare in mind that the 6600GT's core was built for the PCI-Express slot but has been made available for AGP.
http://www.hardwareanalysis.com/content/article/1755/ Read a bit about it here and research it a little.