If you are using iBook and iMac, the fastest interface you can use is Firewire 400.An FW400 External Closure with a 7200rpm drive would be faster than the Bulit-in HDD.Max Speed about 37-38mbytes.If you are using Macs with a Firewire 800 port, (PowerBook, MacBook Pro, PowerMac),get a FW 800 enclosure. Speed could be up to 72mbytes.
A little bit addition,I should say, a FW 400 3.5" 7200rpm external HDD would faster than the iBook's internal HDD, but not the one of iMac.Here is a list of enclosure that can boot OS X.http://mac.hk/appletalk/viewtopic.php?t=12416I am now using the Maxtor Diamond Max 10 300GB with a Firewire 400 Enclosure with my PowerBook.