Ah, I remember that board- hacked a few of those back in the day.
Sophie, is there an Extensions.mkext file? If so, have you tried trashing it? If you have it, that file can get corrupt- it's the file that the system actually reads the kext extensions from. It should be rebuilt on restart.
If it doesn't suffer the problem in safe mode, then it is likely related to an extension problem. An old .mkext file would be my prime culprit.
I would also have guessed permissions/disk repair. Also make sure the disk isn't almost out of space.
Also, you might want to try the ol' Maintenance automater script:
http://www.apple.com/downloads/macosx/automator/maintenance.html
If all else fails, before you delve into a new motherboard, why not just clean re-install the system? Sometimes, that's just the easiest/only way to go. (Yes, EVEN the supposedly infalible OSX!)
I need to borrow back a system I built for someone still running 10.5.8 on a DG31PR and see how it goes installing Snow on it. It's definitely possible- there are a few guides floating around the net, and the board is listed on the osxProject HCL wiki up to 10.6.3.
If it did come down to replacing the board, the Micro-ATX/775-based board I'd most recommend is the GIGABYTE GA-G41M-ES2L:
http://www.newegg.com/Product/Produc...82E16813128388
Runs Snow Leo very well, and there's all manner of methods of setting it up: IE: kakewalk, Tonymac, etc.
Note: I don't know about the GA-G41M-Combo version of this board- it has DDR2 or DDR3 RAM, but I have no idea if it's still Hackintoshable. Comparing the two boards on newegg reveals some major changes in the chipset, LAN and Audio hardware, so if it does work using the same procedures as the non-combo, it'll probably require some kext-hunting.
I don't know of a 775 board with compatible onboard video without regressing backwards to some horribly outdated Intel GMA9XX based board; there's just no advantage to it vs. even the most entry-level compatible video card.