The fastest I could find is the G4 "MDD" tower: https://everymac.com/systems/apple/powermac_g4/specs/powerma... ... Though the 800mhz iMac G4 is a nice alternative (which I happen to have): https://everymac.com/systems/apple/imac/specs/imac_800_17_fp... Makes for a great Mac gaming machine since it is pretty fast, looks great, is a fully self-contained computer with display, and can run both OS 9 and OS X.

Looks like the most truly-OP upgrade you could do is a dual-1.8ghz G4 upgrade card which is kind of crazy to consider: https://www.sonnettech.com/publicfiles/pdfs/pdf_datasheets/d...

You can get a 1.67Ghz G4 Mac Mini on eBay with OS 9 preinstalled. It's wicked fast (especially with an IDE SSD adapter)

There's faster single thread accelerator cards that were made for a while by a guy on the 68kMLA forums, but that was many years ago. I think Action Retro on YouTube has one that broke 2Ghz