Here is the strange thing about the 4850. I just bought one and I have the same processor as you. I get 60 constant will all filtering turned up. However, sometimes when I load FRAPS the FPS drops like a rock, but not all of the time. Closing FRAPS and WOW and restarting both restores FPS to normal levels. This happens in other games as well. I don't know if this is a normal FRAPS issue or not since I never used it with my Nvidia card. Are you using FRAPS for your FPS measurement?
For some reason I'm getting anywhere from 15-75fps with a 4850 in World of Warcraft, I would of thought that I could easily stay minimum 60fps with such a card and I've heard others doing better with lesser cards. I get better fps in bf2, fear and unrealtournament3 with everything fully maxed. Whats going on?
4gb ddr2-800 5-5-5-18