|I have a couple on mine, but it's mostly ports of PC games, and the system's a PowerBook5,6, so 15" 1.67 GHz. Still, let's see, in case it helps... maybe regarding performance at least, since that's something I found very inconsistent when trying out games on my machine(s).
Warcraft III and its expansion run very well, the recommended specs for it are a 600 MHz G3 or G4, with 256 MB RAM and 32 MB VRAM. SimCity 4 isn't too terrible, either, but certainly runs slower than Warcraft - it requires a 700 MHz CPU or higher, but does recommend a G5. Quake III, either retail or using the ioquake3 engine, runs very nicely as well, unsure about the specs, tho.
Halo is... well, it honestly runs like crap on both of my G4s. Minimum requirements (for the version I have; there's been a later, PPC and Intel release that might differ) state ex. a 800 MHz G4 or G5, but in reality, it chugs along even at double that clock speed. It's playable, especially when turning the graphics settings down, but it's not pretty (literally). Similar situation for Unreal Tournament 2003.
There's also a port of Puyo Puyo Fever that's very playable on my PowerBook, tho IIRC, it does stutter in certain situations (ex. resolving big chains, Fever mode). No idea on the requirements.
Finally, something I used to play online sometimes a long time ago on Windows and recently rediscovered, Armagetron Advanced also has a OSX PPC version; runs really well, too. The official site seems to be down right now, so the link only goes to Wikipedia.
Official game releases or otherwise native games aside, ScummVM on OSX PPC works well, too. I played through most of Curse of Monkey Island on my G4 tower (1.6 GHz, 1.5 GB RAM) last year or so. There's also various emulators that still run on PPC, like Snes9x, but you'll probably have a much better experience playing these games elsewhere.
Pretty sure all of these require OS X v10.4 or lower, some as low as v10.1, none v10.5.