The Powerbooks are sleek and pretty powerful, for a notebook. Quiet, small, well-built, and a pleasure to take on the road. The Powermac G5 is a massive, noisy, behemoth. That said, a dual G5 will stop its hiney in Photoshop and video applications. There's no comparison speed-wise, and it's not just the processor speed. It's also the RAM speed, the bus speed and the hard-drive speed. There's many many engineering differences that make a Dual 2.0 more than twice as fast as a single 1.5 at most tasks.
Also, consider Powerbook max. RAM = 2GB. Powermac G5 max. RAM = 8GB. Hard drive is also twice as big.
CV