Hey, I just had a thought. I ran into a strange ass problem the other day, where after installing the nvidia proprietary drivers for my 6200 agp card, X wouldn't start up. I have no idea why this is, but come to find out, the nvidia module was being loaded during startup, and that prevented X from starting up. I removed the module from memory with a
rmmod nvidia
And X started up perfectly using the same proprietary drivers I just updated with. That reminds me again, since you can get to a shell prompt, have you downloaded and tried updating your nvidia drivers? I had to do this to get into AVWizard when switching from an ATI card to an NVIDIA card without re-installing.
Oh, I ended up adding 'rmmod nvidia' to one of my startup scripts, box has been running great for weeks now.