Imac G5 Vs Powerbook, Do I lose a lot of power? Options

[camarao]

I´m thinking to switch from an Imac g5 ( that noise is so irritant...) to a G4 1,67hz Powerbook 15 ". I use only Logic pro 7 and Kontakt.
Will I lose a lot of processing power? Do I need to do a lot of freezing in Logic? Is a 5400rpm hard disk enough ( for a maximum of 16 audio tracks) ?

Thank you