I have bought two external drives, one USB 2.0 (160GB) and one Firewire/USB 2.0 (300GB) Both are very good, all things considered. Do think about a larger drive than 100GB though, as the price differential isn't great and you can never get anough storage. What is adequate today, will be a bit on the small size tomorrow, depite your best efforts.
Units by major manufacturers might give you extra piece of mind, but usually include decent backup software for the few extra quid in cost. They still tend to be preformatted in FAT32 for compatibility with earlier machines, but I prefer the NTFS format, which is used with Win2K and XP.
Just as a horrible wake up, data can be lost, so I use mine as a backup rather than just an archive and therefore have separate copies on different hard drives. I sleep better that way.
Conan