ADS Technologies makes a good external interface USB 2.0 drive. The hard drive inside is actually IDE, so its not going to be any faster than an internal IDE drive, but it is just as fast.
We have a Buslink usb2 80 gig drive. Seems to work great. It just took some time for me to remeber to save everything there because Vegas wants everything to go in the Vegas folder..but once you get it all changed over it's fine.