Firewire
If your in the market for an external drive and your computer has Firewire 800 then do yourself a massive favour and pick up an external drive which has Firewire 800 too. My old external USB couldn’t cope with backing up the desktop and laptop so I invested in a 500 GB My Book Pro and the speed difference is very noticeable.
Looking at the specifications for USB 2 & Firewire 800 tells you that Firewire should be faster but real world difference for me was that the backup was a fifth of the time using the new drive against old - much faster than I was expecting. Speaking of the drive I’m fairly impressed with it as it’s quiet and has a nice finish too it. It’s also easy to power down unlike many others. I also intend to use the drive for Time Machine but only on certain key folders.
I had another thought on Time Machine - not much use for laptop users unless they are attached to an external drive, which aren’t always portable or convenient.