A possibility occurred to me that could easily cause all of your problems. As I recall the power supply in the sawtooth model is not the most robust. So your newly added 1½ GB green drive may be pushing the power supply to the edge or even beyond its limts. That could result in low power to all your drives which in turn would cause all sorts of read/write errors. The only test would be to remove one of your drives, not necessarily the 1½ GB green drive, just one on either the ATA or SATA bus. Personally I would start with one of the SATA drives.
As to the Apple Hardware Test, unfortunately the downloadable AHTs Doni referenced do not cover the G4 Sawtooth. However the freeware
Rember or the $1.39
Memtest are excellent tools for testing RAM. In either case be sure to run for at least five fully test cycles or until a memory error is detected.
Unfortunately RAM is only one of several possible hardware components that may be causing your problem.
TechTool Pro ($97) or TechTool Deluxe (that comes with AppleCare) both have extensive hardware diagnostics, but whether or not they are the equal of AHT I don't know. Certainly they are capable of detecting gross hardware errors in the system.
