I guess the reason is because they are some hot cards. Heatwise that is, plus I have been using Nvidia cards since my first build back in 97`. I have a laptop now that has an ATI mobility X1400, and I do not like the drivers. Guess I am just used to Nvidia.
If you do get the 8800, your CPU will likely be the bottle neck.
With that mobo, wait to start to OC, make sure you have a decent PSU, and cooling and a mobo that OC friendly. The PSU is big part, cause you can fry your whole rig possibly if you use a cheap PSU. I had one go out on me when I was OC`ing the first time, luckliy it didn`t take anything with it.