You didn't mention if it is a S900 or J700, etc....
The last round of S900 towers were actually sold with Newer Tech G3 upgrades and whoever had the Umax before you probably just returned the original processor and kept the upgrade for another machine, or sold seprately, etc.
Newer is gone but Sonnet and XLR8 still make G3 and G4 upgrades and now XLR8 has a DUAL G4 upgrade! So, do some research, you can still get some great performance out of that legacy machine.