The iMac will be faster in all respects (disk, video, processor especially), and by a large margin on processor and disk-related tasks. The MacBook will be faster than the PowerBook processor-wise, but for disk-intensive operations it'll be about the same, and graphics won't be any better. 3D graphics will be much slower. As for iMac vs. MacBook, processor-wise, the difference isn't big, but the iMac has a substantially faster disk, which may make a big difference when it comes to video-editing and the like. Depending on the app, the graphics card will also help.
What will make a big difference for the MacBook and iMac is native apps: Adobe CS3 is vastly faster than CS2 on Intel Macs.