After much dispare, (and a few lockups on Doom3 running XP) I had a look in the other forums for 6800 ultra problems. It seems that many people are having problems from many different card providers.
HOWEVER, many of the board vendors have issued flash-rom updates for their cards, my POV 6800U was one of them. After flashing the card bios my machine is stable under both linux and windows. This makes my machine usable now w/6800U. Furthermore, many graphics errors that were visible in Doom3 and other games have now vanished.
So, if you're having problems, look at the overclockers forum: http://forums.overclockers.co.uk
under graphics. There are a lot of details there for sorting your GF6800U out.
I hope this helps!