Quake II and 3 along with Myth II ran just fine on my G4. Did you get a bad mix of ATi drivers for your card?
If you somehow ran an older ATi driver installer you could have wiped out the *new* drivers. If that isn't the case, try going *back* to the older set of drivers (if you have Quake II they have a universal installer for ATi on their CD).
[This message has been edited by WarDawg (edited 04-11-2000).]