I had a chance to try a 566 Celeron from another pc I was working on. It is of coarse a Coppermine core unlike the 433 that the rig started with.
To my surprise...it worked!
I put the 900 in the other guys rig, and they both got an upgrade 🙂.
Now I wonder if the original rig the 900 wouldnt work in, would take a 766 (the last 66fsb chip) ???