how fast a disk you'd put into a USB or FireWire enclosure is quite irrelevant...
You can choose a udma-133 Maxtor, fastest IDE disks on the market...but the USB- and FireWire-to-ATA bridgeboards in the enclosure generally aren't capable of anything above udma-33...
YMMV and there may be something faster out that I've not seen yet.
As for bus-powered drives, you'll have to plug in a brick for a 3.5" desktop drive, since they draw 12v as well as 5v juice...and USB and FireWire only pass along 5v...so if you want a bus-powered external drive, it'll have to be a 2.5" (laptop type) drive which only uses 5v power.
Oh, and if you have both FireWire and USB (even 2.0), definitely go with FireWire. It consumes less CPU cycles to perform its I/O operations, thus bringing less overhead on your system.
I own at least a dozen external hard disks. One USB2, one FireWire, one combo, and the rest are all SCSI...
Oh...and don't forget that iPods and similar devices are also external USB drives! (I have Gateway's knockoff of the iPod...has a 20gb disk...if you're looking for "a good excuse" to get an iPod...hehehehe...
