It sounds like maybe you are getting CPU throttling issues. Have you tried reseating your HSF, and maybe cleaning it and replacing the thermal interface compunds with something better, like Arctic Silver?
I think there's definitely something other than the video card limiting your performance. I have a 6600 GT and it gets ~3700 in 3dMark 05, and the 6800 should score at least as well as it, and really it should score even higher.