Don't get a 6600gt
The x800 gto is the same price, much faster, AND can be easily modded to enable the extra 4 pipes so it's basically an x800xl (as long as you get the saphire one).
It will be a bit more expensive than x800 pro/9800pro, so if you don't want to spend that much, look for a cheap 9800 pro on ebay or fs forum.