I had real problems with my 4850 on Vista 64, and seemingly not related to the heat issues. I'd created a profile to adjust the fan speed, which worked well for keeping the temperature at a sane level, and it didn't help.
Essentially what would happen is the display driver would crash or stop responding. Sometimes Vista was able to recover, and other times not, and the whole system would go down. This happened to me both in games and just sitting at the desktop, regardless of gpu temperature. The problem may have something to do with using dual monitors based on various posts around the internet, but I could never figure it out. If you're using just one monitor, you might have no problem at all. If you want to search for the particular issue, look up "atikmdag". There are a whole host of fixes proposed, but none of them worked for me.
Many of the things I read indicated that it's a problem with Vista itself, and that the nVidia drivers can do the same thing. Despite that, I've never had it happen with the 8800GT prior to switching to the 4850, or with the 8800GT after switching back from the 4850.
Kinda sad, I wish I could figure it out, because the 4850 performance was very nice.