FWIW, here's one more approach . . . .

I back up three computers. My home desktop, my travel/client-site laptop, and Mrs. Cache's laptop.

I have two Western Digital Passport drives and have installed the freeware version of "SyncBack" from w w w . 2 b r i g h t s p a r k s . c o m on each system.

SyncBack does an intelligent backup . . . it only copies those files that need to be copied which means that your USB 1.x will be less of a problem. You can configure it to backup (one way) or "synchronize" (two way).

Why two external drives? Offsite storage! Don't think any of the other posters mentioned this -- depending on the nature of the data that you're worrying about, it may be appropriate to include "offsite" in your back up scheme. The WD Passports are small enough to fit in your safe deposit box (or your office desk or where ever). You do your backup, remove the drive; take it to your offsite location and exchange it for the "other" one.