Hard drive.. internal or external?

[mgarzag]

I have a macbook (2.4.. 4GB) and using pro tools m-audio

it's been working "fine".. sometimes i get the CPU overload with not many tracks, specially when using plug ins like eleven or ez drummer..

i read that is better to have all your audio files in an external hardrive, and let the internal handle the application... so when you playback.. is from the external hardrive

I've started to use more tracks.. and sometimes it wont support the plug ins so i have to print to audio the midi tracks... so I don't how many tracks can my external hardrive firewire 400 can handle..

Ultimately.. i end up with a session of 20+ audio trackas before mixing.. then i add eqs, compressors, gates, reverb if needed..

my question is... can an external hardrive handle 20+ audio tracks? or should i get another internal hardrive... so transfer time will be faster..

on the other side.. if thinking of buying one of the new imacs, they look nice... but i don't know if the firewire 800 port will support playback for 20+ audio tracks... also, the new imac doesn't come with sata, so fw 800 is my only choice

any comments? suggestions?
thanks in advance