Thanks for posting this, it's one of the reasons I don't think next gen cards are quite worth it just yet (not to bash your hardware of course, I'd much rather have a 5970 than what I have now). My 4850 oc'd to a 750 core clock was no match, I had to play at 1024x768x32 on high settings just to get decent frame rates. Of course I'm stuck on a 512mb frame buffer, but you don't have that problem.
Your 5970 can effectively double my cards output with ease on average and you still see performance issue's. Doesn't make any sense.