I think my experience with gtx 460, reported here http://www.nvnews.net/vbulletin/showthread.php?t=156372
, can help a little with diagnosing the problem.
Notice that offending card was causing freezes on one motherboard, but on another it caused various xorg errors, always preceded by "NVRM: os_pci_init_handle: invalid context!" flood in sysylog.
"Problem" card was ASUS ENGTX460/2DI/768MD5, "freezing" mobo ASUS M4A87TD, "nvrm" mobo ASUS M2N-E