"Fastest" also depends on whether you are talking PCI express, SLI or AGP. In the high end cards like the Nvidia 6800's, or the ATI x800 cards, PCI express is faster than the comparable AGP version of the same card, and SLI is even faster yet. Talking a game like Doom3, Nvidia pretty much takes it. But the you go to one like Unreal Tournament and at the highest resolutions and "eye candy" settings, the x800 just edges out the 6800 (in AGP anyway, in PCI express/SLI its just the opposite).
Its mostly a matter of personal preference. I prefer Nvidia. There are lots of guys who are pro-ATI. I hear people all the time say stuff like "oh, I may have to switch, so and so holds the edge right now". They've been see-sawing back and forth for a few years now. Bottom line is, research your buy and go for what you like. If you buy an Nvidia card, and its the hottest now, in 6 months ATI will have one thats faster than the best Nvidia card. Or they will put out a better driver. Whatever. 6 Months after THAT, the other one will be on top again.