Dude, that's completely out of the price range. everytime I turn around I see you suggesting the Ultra to someone that aren't looking to spend more than $200....that's not the save all answer.
The 6800 doesn't have GDDR3, but I honestly don't know how it compares to the 6600GT other than I simply heard it sucks.
You can mod the 6800 into a GT and with good cooling hit ultra speeds, but that's a really long and painful modding process and of course it may not actually work out in the end.
Those 6600GT's are pretty damn cheap. I went with eVGA was still $199, but I trust in their quality so I went with em. Have heard of a lot of people having troubles with XFX, don't know about gigabyte though.
Tough call honestly, although after getting my 6600GT today I'm a bit biased towards it