You're always going to get better performance from a Mac Pro. But if you need portability, I've used Motion 2 on my 12" Powerbook G4 with a 64 meg graphics card and 1.26 GB ram for short projects--the only limitation for me was hard drive space. Rendering is slower and less can be done in real-time, but it beats hauling around a Mac Pro for me.
I'm sure a dual-core machine with more ram and a faster GPU (Macbook Pro) will give you acceptable performance. If you want really good performance, you need a desktop.