I think the new 17" Powerbook will work very well with Photoshop, Maya, and Shake. However, XSI will not run on it, because there is no OSX version of it. As far as performance between the G4 and Pentium 4 goes, you're going to find different reports and opinions as to which one is faster. IMHO, if what you're looking for is pure speed and power, then go with a Windows-based notebook. In general, 3D applications run faster on Windows, and you have more options. However, if you want a pleasant user experience (both in terms of the OS and the hardware), go with the Powerbook. OSX is a joy to use and makes XP look dumb and convoluted (IMO), and the physical design of the Powerbook is all about elegance and usability. And, yes, you can use any 3-button USB mouse with a Powerbook (I've used a variety of Logitech mice)

Realistically, it boils down to personal preference and what kind of applications you need to run. If you have to run XSI, then your only choice is a PC notebook. Otherwise, the options are about the same on both systems.

FYI, I currently use an 800 MHz Powerbook G4 (15.2" screen) for all of my 3D and video work, and it has been a wonderful experiece. I still keep a PC around for anything Windows-specific, but I use my Powerbook about 99% of the time.