I've gone from a Radeon 9500 to a 6800GT, and I have no regrets. If you're at all interested in HTPC, the nVidia cards (combined with their $20 codecs) are by far the best. For gaming, they also have advanced support for shaders that ATI skipped, although only a few games so far have been announced that support it. Far Cry and Battle for Middle Earth are the only ones I can remember off the top of my head. Two identical nVidia cards, when combined with a motherboard that has two PCI Express slots, can use SLI for the ultimate, if you really have no price limit. I'd go for whatever the fastest 6800 flavor is, although from what I hear PCI Express might be a little bit better.
One note about noise: You can get the Arctic Cooling NV5 Silencer for better temperatures and much softer fan noise, but you lose a PCI slot.