go for it...
I've never purchased a video card from nVidia, but I did some research before picking up my 8500. At the time, it beat the GF4 in several tests, enough to give it a 10 out of 10 rating from the magazine. The GF4 didn't rate that high.
I would suggest getting it from someplace with a good return policy (user friendly). See if they will take it back, without a pesky restocking fee, if it doesn't perform as you need. They should allow it, depending on who you buy it from.
It would be interesting to get some real world comparisons between the two cards.
Has anyone else noticed that as soon as ATI releases a great card, nVidia scambles to make theirs better?? At this rate, as soon as you get one, the other company will come out with something better within a few months. The cycle never ends....