Hi,
I'd steer clear of ATI's 9600 series(way too last gen). The 6800XT is a cut down 6800 and will probably peform less than a 6600GT, but not by much. The 6800 is faster than the 6600GT but again not by much making the 6600gt way better value.

Personally i'd go with the x800gt. It has a R480 based core(x850xt/pe)so i expect it has fair bit of overclocking headroom. Also its the only card that you've linked with 256mb of memory which will help alot in newer games.

If i had to buy one of the Nvidia cards i'd go with PNY Geforce 6600GT since its probably got the best price/performance ratio.
GL