I did a simple test using a FW400 g-tech drive. The drive supports both FW400 and USB2 connections.
I created 9 copies of one 600 MB DV/NTSC file and imported them into a FCP 7 DV/NTSC project.
First, I created a 4 file multiclip. The drive would play it without dropping frames using either FW or USB connections
Second, I created a 5 file multiclip. It would play fine in FW400 but dropped frames in USB
Next, I created a 6 file multiclip and it would play fine in FW400 but dropped frames in USB - to the point of not being editable.
I kept adding files to the multiclip - the firewire connection would play a 7 file multiclip ok but 8 streams choked it.
What does this mean? Firewire supports a 50% higher effective bandwidth than USB2.
In the "for what it's worth" category, I could run 16 streams of DV off of an internal 3 drive RAID 0 array on an MacPro without a problem.
ymmv,
x