I can get 96+ tracks at 24/48 through a USB2.0 connection before having problems with dropouts.
I've heard that firewire is generally prefered over USB because USB uses more CPU resources.
My external for my portable setup is a 320GB WD Caviar (I think it's even IDE or ATA), Metal Gear Box, mounted in a swap tray. The computer is an ASUS notebook. I haven't done a full, large scale project with the setup yet though.