Are you going to use Firewire or USB? If only USB then go for the Essential version as it will save a little money. The Premium ones also have a second light but who needs that.
You also do not need the backup software if using XP. XP will recognise it as a drive with plug and play so you can just drag and drop, copy and paste files.
A couple of things to be careful about is speed and the power source. Speed wise, you won't get internal HD performance maybe about 30-40 megabytes a second. Internal are rated at around 100. If you are just going to use it for backup then should be okay.
The power source is an adaptor and although these are reliable, won't last for ever. Probably last as long as the drive through.
Anyway, I have the 320GB version and four internal HD's including two DVD burners. I'll probably get the 500GB version next 