Thanks for all your comments.
I think I'll suggest to my sister that she gets two 1GB Western Digital My Passport Studio drives (I've read a couple of reviews and they seem a good buy) where one is used with Time Machine for hourly backups while the other one is used for off-site archive backups (in case of theft, fire etc.).
I'm not sure what would be the easiest/best software to use for the archive backups but Carbon Copy Cloner shouldn't be such a bad choice, right? The free version still works with 10.6 Snow Leopard which she has, so she won't have to buy the current version which is US$ 40 IIRC.
Yes, USB would probably suffice, but since she uses those ports for other things then Firewire 800 would be better in my opinion.
As for the bus-power problems mentioned in this thread: I've personally got problems I believe relate to that, but that's with a 1.67GHz Powerbook G4 and multiple Firewire drives and not just a single drive as my sister will be using. My Powerbook has a bus-powered
G-tech Mini drive attached to the Firewire 400 port and a
Proavio S4UF (4-bay hard drive rack) which goes to the Firewire 800 port. For some reason file copies run really, really slow at times when using both drive enclosures (I usually just have 1 or 2 drives in their bays running at once with the rack), so in cases like that I have nothing else to do than physically switch off the G-drive Mini (there's no response when trying to unmount it from the Finder) which of course results in a warning/error message about unmounting it incorrectly. But all of a sudden the Proavio drive continues copying files again (what's really strange about this is that just the G-tech drive is bus-powered. The Proavio enclosure is AC-powered and also runs on the Firewire 800 connector, but I think I've read somewhere that Firewire 400 and 800 share the same data-bus hardware).
It could also have something to do with the Proavio containing Western Digital "Green" drives which automatically spin down after a very short period of time according to various threads on the subject. Apparently this is a Western Digital Firmware feature, but I'm not sure if Mac users are affected or not. In any case WD only supplies a firmware utility for MS-DOS, so there's not much I can do about it.
But my sister likely won't have to deal with this kind of stuff using just a single external drive. There should be plenty of power available for that on a Macbook Pro.