If you're an ATI head, the 9800XT is the top of the tree at ATI (for the moment).
If you're an nVidia bunny, the FX5950 Ultra rules the roost.
For less than pipe-hitting hardware, I honestly couldn't comment... what's the point in looking anywhere but the bleeding edge?

And in the unbiased world I inhabit, I don't mind admitting that the ATI is faster than the nVidia, but as I've had problems with the ATI cards I've owned (most recently a 5700), just as soon as I've saved the £300 for it I'm getting myself an MSI FX5950 (to go with my loverly MSI mobo)
EDIT: And of course, both parties have their next-gen technology waiting in the wings - nVidia are expected to announce NV40 a fortnight or so ahead of ATI announcing R420... and with luck the current high-end cards will begin to dip in price sometime soon after...