I don't know if this helps, but Barefeats did a rendering comparison test and it looks like the Powerbook is really hampered by lack of processing power. Here are the results:
I'm not sure if realtime performance is more based on graphics processing or CPU speed, but if it's CPU-intensive, the dual-processor systems seem to have a big edge. If it's primarily graphics based, then the Powerbook should do pretty well since the 9700M is a good performer.
What video card do you have in your G5?
PS: [SHAMELESS PLUG] If you're looking for a good deal on a 17" powerbook, I've got one for sale. It's only a 1.5, but I don't think there's much difference going to a 1.67... [/SHAMELESS PLUG]