Firewire lets you do things like [link|http://docs.info.apple.com/article.html?artnum=58583|Target Disk Mode]. I don't think you can do that with USB2.
I'd probably get a [link|http://www.newegg.com/Product/ProductList.asp?N=2010090092&Submit=ENE&Subcategory=92&Description=firewire+external&Ntk=all|Firewire external enclosure] and install my own choice of hard drive. Choose carefullly. :-)
On the software side, [link|http://www.bombich.com/software/ccc.html|Carbon Copy Cloner] is something that I'm trying to use with out Ti G4 Powerbook and G5. It's not yet a Universal binary for Macintelitoshes, but that's only an issue in trying to get the OS backed up between PPC and Intel. It apparently runs fine in Rosetta.
HTH a bit. Note that I haven't yet done what you want to do, so sprinkle liberally with NaCl...
Cheers,
Scott.
(Who recently got an 80 GB disk for this purpose, but a USB2 enclosure...)