Haha, you're all wrong! =)
I have a 5900 (128MB) and a 6600 (256MB) and have benchmarked them side by side in the same comp. The 5900 ALWAYS wins by a comfortable margin (20-30%).
I think the 9800 pro would likely win.
The x600 would probably be last, x800 is a wonderful card, the x600 is a piece of crap.
So, I'd say: 9800 pro, 6600, 5900, x600.
Personally, I always buy nVidia (mostly for linux reasons). In linux, the order would be 5900, 6600, 9800 pro, x600.