I've got a GigE/Mystic dual G4 now (thanks, coius!) and stuck in a couple 256MiB PC133 sticks I had. I run NetBSD on this machine, and while it sees these, it doesn't use them. The main difference, AFAICT from the dmesg, is that the working stick is double banked while these sticks are single banked. I used to have those sticks in a beige G3, and they worked just fine there.

Just another possibility for your consideration.