I personally would also recommend that you use the first system restore disc and restart the mac, holding down the 'D' key. This will let you go to the system diagnostic menu and run a hardware test to determine if any of the ram is bad.
I once had an issue with my Powerbook where the ram seemed fine while in OS X, but copying large files and installing programs would error or crash. It turned out when I ran hardware test, that the ram was bad and was corrupting the data. T_T;