For now, stick with Geforce. The Radeon cards dont support Pixel Shader 3 which is a big deal. Also, the new game F.E.A.R., doesnt run very good on the Radeon cards.
There no comparison between a geforce 6800GT and a radeon 9800 pro. THe geforce 6800GT destorys the 9800pro. It really not even a fair comparison since the 9800's was the best of last generation and the 6800's are the best of this generation. (some might consider the 9800's 2 generations behind and the 6800's last generation)
No, there is not. The 7800GTX gives a whooping on the 6800 GT, and the 6800 GT DESTROYs even the highest end Radeon X850 cards. For high end, there is pretty much no competition, Nvidia is the way to go.