Have to buy external HD for a friend's iMac. Any reason to buy one bigger than her internal drive?

[Spiff]

Don't boot from USB. In the past (pre-intel) Apple has crippled the USB port on their machines. Even though theoretically USB is as fast as FW400, it ain't on a Mac. FW was much faster.

I admit to ignorance on the intel Macs, but my suspicion is that Apple has continued the tradition.