I think it has very little to do with the graphics card. As all the games like Battlefield 1942, Call Of Duty 1&2, UT 2004, etc. run like a charm in full resolutions on the G5. Civ IV is clearly very CPU intense and it seems that the code is everything else than optimized for the PowerPC, as Civ IV runs hundred times better on a older pc with an older graphics chip with only half the VRAM...