I'm running motion on my 17 in. powerbook with 1.5ghz and a gig of ram memory. It works just fine. Real time playback is ok when you're not running too many layers/animations, however, when you start loading up, RT playback becomes pretty much non-existant. My Powerbook is my sole workstation and it pulls through with flying colors (then again i'm only 18 and i dont have the time constraints of most professionals). Hope I've provided some useful info for you. :-)
Cheers!
~James