To me, one of the prime benefits of going with Nvidia has been stability in games et al. I don't recall ever having any big stability issues with anything they made, and this is with around a dozen different products over the last several years.
I've heard about there being some issues with Vista, and I hope that's just a fluke. I haven't encountered anything to date, but I don't run Vista as my main operating system on anything. (And I honestly don't feel any real need to run Vista until I'm sure it's going to work as well as XP and provide SOME non-aesthetic benefit.)