Do you get anything on the console at all? It might be worth leaving it on a text virtual console (rather than X) overnight, and if it's hung, then try and issue a few "Magic SysRq" combinations to see just how hard it's hung. You might need to enable these first, not sure if Debian and friends enable it by default - "sysctl -w kernel.sysrq=1" should do it.
To issue the keystrokes, hit "Alt-SysRq-<letter>" where the most important ones are as follows:
- m - dump information about memory allocation
- t - dump thread state information
- p - dump current CPU registers and flags
- c - intentionally crash the system (useful for forcing a disk or netdump)
- s - immediately sync all mounted filesystems
- u - immediately remount all filesystems read-only
- b - immediately reboot the machine
- o - immediately power off the machine (if configured and supported)
You'll probably want to look at t (useful to see if it's alive - you'll get LOTS of information on the screen, and it can be used to see what's died), s to sync and u to unmount filesystems (in that order, ideally)... and "b" and "o"... any possibly "c" :) You've mentioned you're running the nVidia driver - I'd get that out of your kernel as soon as possible - it's quite likely that's the culprit... it's had a few nasty bugs in the past where it just randomly feels the need to scribble all over bits of kernel memory it shouldn't be touching, and Bad Things(tm) tend to happen.