There are some very good open source games that are Universal Binaries. Check out some like Tremulous, Sauerbraten, Neverball, World of Padman, and the like. They run pretty well on my 2.0 GHz iMac G5 17" ALS.
I think there are quite a few good games for the PPC architecture (in fact, I can probably think of more problems the other way round, with games not working on Intel or not playing as well under Rosetta). So the good news is you should be fine.
Alien vs. Predator
Alien vs. Predator 2
American McGee's Alice
Call of Duty (and Call of Duty: United Offensive)
Quake III Arena (Open Source version, ioquake3, too)
Rise of Nations Gold Edition
Star Wars Battlefront
Tom's Clancy's Rainbow Six
Wolfenstein: Enemy Territory (free and Universal binary)
I'm surprised nobody has posted anything about the original Fable. I have been playing The Lost Chapters on my G5 for a while now and I love this game. It's a bit funny that this was one of the games that said "Only On XBox" on it's original case. If you E-Mail me I can send you a link and instructions on how to install it properly. And this is just me being honest: 2 and 3 are not nearly as good as the first.
Check out macgames.co.uk - there's lots of games for both intel & PPC macs. Especially look through the discounted stock page, as those are mainly older PPC games that are being sold off cheap. Not many left now though - seem to be selling off quickly!