My problem seems to be the same as the one in http://www.nvnews.net/vbulletin/showthread.php?t=61651
It appears to be an issue between the ATI RS480 chipset and the Nvidia card. Does this problem only occur with the 6200 based cards, or do the 6600 show it too? In the thread I mention, there is the suggestion that the 6600 cards work.
Windows is stable, so my guess is that there is some interaction between the Linux kernel, the Nvidia driver, and this hardware setup (ATI+Nvidia).