6800gt all the way, 16 pipes with the price of 12 (x800 pro), you can overclock it easy to ultra which is $100 more expensive. Heck, the Gainward Golden sample even pre overclock it for you, with warranty!
However, I will stick with 9800xt. As I usually upgrade every 2 generation. You'll see the difference but for me it's not worth it.
Think of it as getting a car with twin turbo to replace your single turbo car. The twin turbo gives you double the power, but you don't feel it except under extreme circumstances.
My advise. wait until the next generation comes along. You can then either pick the newer card or get 6800gt cheaper