unfortunately, iBook, PCs jsut dont do it the same, on top of the fact that the applications are far better on macintosh platform then PC. but if you must have pc, research specs, and get one with a well known, good audio board, when your blaring stuff over a venues PA, poor sound quality just isnt acceptable, and of course, make sure its got enough horsepower to play everything your doing seamlessly.
I'm not speaking from personal experience, but i know 5 bands (2 of which i jsut saw last night oddly enough) that use macs as thier backfill and effects machines on stage