Ive just built a new games pc, a Phenom 955 BE with a AMD HD6850 etc. I can run pretty much all games on full, but with anti aliasing on, frame rate drops considerably. Is this normal in new games? Would i need a Crossfire GPU to effectively run anti aliasing in all games?
AA has a known performance impact, yes. You could try a less intensive AA setting (4x as opposed to 8x or 16x). Also, there have been some AA-related problems with Radeons over the years, so make sure you keep your drivers up to date.