It might seem like a biased answer but here we go anyways. Steam has recently introduced their new MAC compability program. In other words, you'll find a great list of games compatible on the Mac and for cheap. That said, they only provide digital download (as oppose to DVD or disk options). So make sure you read their end user agreement and that you understand what they offer.
To be honest, I doubt that any reasonably modern game will run satisfactorily on this Mac. It has a very low-powered processor (and it's only a G3) and is woefully short of memory.
On my Mac Mini, with a 1.4GHz G4 and 512MB memory any games that I have tried are unplayable, so with less memory and a processor that is less than a fifth as fast I don't rate your chances. And that's not even considering the graphics card!