Disclaimer: I like nVidia, I was very disappointed in them with the early FX series, but I trust their drivers. I have never been disappointed in an nVidia card I owned. I was very pissed off and angry about the ATI i owned (6mo of no functional drivers do that to ya.)
I'd go the 5900 Ultra route personally. Now that nvidia has put out good drivers for the fx series and it can compete with ati in image quality, speed, etc... I would go with nVidia even if ati had a slightly faster card. Just don't like ati's drivers... even though they are supposedly alot better now than they once were.
Shadus