I will throw out a suggestion that while there's no harm in trying, you will likely find heavy 3D games to be a somewhat less than satisfactory experience on a Pismo. That's not to mean that there's nothing out there-quite the contrary as there are tons of great games that are a lot of fun and a Pismo is more than enough to run.
I'd just avoid things like RTCW, Doom, and Quake III. I have a couple of computers I've built specifically for games like that-I start with a G4 tower, max the ram, add in at least a moderate CPU upgrade if it's not over ~1ghz, and the preferably put in a Geforce 4Ti, or if not that the best OS 9 card I can fit in the computer.
I'll also throw in a mention of an old favorite of mine that will play nicely on any "G" era Mac running OS 9-Titanic:Adventure Out of Time. There's also The 7th Guest if you want more of a problem solving/puzzle game.