I saw a Sonnet G4 upgrade 500Mhz for my G3 DT and decided to bite. It arrived a couple of days ago and I spent some time trying to get it to go. Sonnet's instructions don't mention that a CUDA reset may be necessary. Be sure to power off and it may be necessary to unplug the mobo power connector and remove the PRAM battery for a sec. Mine didn't require that, just a couple of tries pushing the button while power down.
Anyway it's in and working with the 768 Meg memory and boy it's a LOT faster now. Highly recommended for those folks who are needing a speed boost for their classic G3s.