I would expect you should be able to play on Medium/High settings since my old PowerBook G4 17" with a 1.5GHz G4, ATI 9700 Pro, 2GB RAM could run on those settings at native 1440x900.
The lowest graphics card for the G5 was a Nvidia 5200 Ultra which is more powerful than the card in my laptop so I think you should be fine, although I was running OSX 10.3 and 10.4 at the time and 10.5 is a bit more resource hungry than them but you should be fine.