My line of thinking is, generally:
If you're mainly a gamer, go with an nvidia card.
If you're more into video (tv tuner/capture, dvd, etc.) yet still play games on a fairly regular basis, go with an ATI card.
At this point in the game, I don't really see another chip company in the running. Sure, Matrox make good cards, but since the G400 MAX... Matrox are good business cards, not for gaming, not for dvd and tv capture.
To give you an idea of gaming performance, I was able to run Return To Castle Wolfenstein at 800x600 on a P2-450 with my All In Wonder Radeon running Win2k. One thing I cannot do with my setup is watch a dvd while surfing (both on the same monitor) without the movie stuttering. This happens when I scroll with the mouse wheel - the system just can't keep up.
I'm planning on upgrading to the All In Wonder 8500, but no way will I be first in line to get it. I'll give it 3-6 months to take care of any driver issues. ATI has sorta shot themselves in the foot with the regular 8500 drivers, but they continue to improve. I think it's smart they released the regular 8500 first before the All In Wonder version - it gives them time to get the drivers to a stable enough point.