My take: Performance-wise, ATI + Nvidia will continue to leap-frog one another. it's happened in the past, and I don't see a reason why it won't continue. currently, I believe Nvidia has the edge.
However, driver-wise, Nvidia has a huge edge on ATI - just compare the features, I'm thoroughly impressed...so, even when performance isn't quite as good, Nvidia currently has a much better driver. Let's not forget that Nvidia absolutely kills ati when it comes to linux compatibility...
A confession: I'm a happy ATI radeon 9000 owner, as 9000vsGF4MX, 9000 wins for what I do, even with those driver problems, I still prefer...You see, I have a GF2MX200, as well, in an old box. the 3d drivers are awesome, considering most of the features still work on that ancient card. That can't be said about a rage128 I've got. Both are getting better, but I hope ATI puts a lot more work into their drivers compatibility/feature-wise, then they have been.