so ive basically got the set up yer describing...
15 inch pb 1.33 running pro tools le with a 002r. a lacie 7200 160 gig fw drive acts as my main audio drive.
now thats bare minimum, and it runs great. i origanilly purchased the 1.25 ibook, due to time restraints and lack of money at the time...and found the lack of a spit (dual screen) desktop sucked... it was not a bad machine with only 256m of ram built in.
so i returned it to get th 15inch pb. once i upped the ram to over a 1 gig ive never had a problem. as the pb's come with a stick of 256... it didnt like only having 256 as it would lock up alot (pro tools that is).

my advice is never edit on the start up drive. it gets to messy.its tempting cuz it will work. to do but resist.
and get a few medium size fw drives mabey one big one to back up Everything on and a few for diff. projects.
honestly ive never had a problem other than speed with a 5400 rpm laptop drive mounted in a fw housing. but thats not 100% reliable.
i love my pro tools set up...and i think u will to.

im running 10.3.5