I've actually been mulling PowerBook G3 Wallstreet PDQ upgrades myself, especially SSDs like this one from OWC:

http://eshop.macsales.com/item/Other%20World%20Computing/SSDMLP040/

They claim it to be 100% compatible with the Wallstreets, and since this is a dedicated SSD and not a CF adapter, I would expect the performance to beat most spinning platter hard drives hands down. It's just that I can't find any benchmarks online for these drives, and no one among us seems to have tried one yet, and the price is $150 plus shipping.