My Wallstreet II 300 runs 9.2.2 and when upgraded to 512, could probably handle 10.3 . I'd imagine that once you get 768mb of RAM, you could run 10.4 without an issue. A 400MHz Pismo with 768mb RAM is the same as my B&W Powermac G3(Yours might be 500MHz, which would be even better) which performs reasonably well. Browsing is a bit slow, but it is quite usable.

The other option is, as coius will vouch, MintPPC. It is the best OS for ANY G3/G4/G5 mac. The developers are active on the forums and can answer any of your questions and solve pretty much any problem. It gets the most out our older hardware.