Well, here's something from my personal experience.
I purhcased the X800 Pro when it first arrived, all $420(USD) worth

Yes it's a very fast card however, given the fact that you can now purchase an X700 Pro from CompUSA for only $200 (AGP or PCIe) and not really notice a performance drop, then I would recommend that one as a very good buy

The reason I say this is because I recently upgraded another one of my boxes which had a tired old nVidia GeForce 4-4600. It was fine for some things but really showed it's age with pretty much any new game title

I was thinking of keeping it nVidia based but I looked around at the 6600 GT cards and pretty much most of them were going for $50 more then the X700 and all the ones I looked at only had 128MB of frame buffer instead of the 256MB on the ATi graphics card.
I think the only reason to go with a higher end card at this time is purely for frame rates and bragging rights
