It's good that you're looking ahead and considering an external drive, as video editing takes alot of disk space, both for editing and storage.
You can use an external hard drive with a firewire connection, which is preferred over a USB-only connection. If you want to maximize the speed, you'll use an external drive which supports the faster FireWire 800 (IEEE 1394b) port your system has.
You might consider building your own. Sounds daunting, but it's really very easy. The benefit is that you can use a hard drive with a 3 or 5 year warranty, unlike most complete units that only have a 1 year warranty.
It's really as easy as this: Buy the drive you want (I'm currently using a 3.5" Western Digital 320gb drive). Generally I've found the Western Digital drives run cooler than others. Buy the case you want (the oxford chip set works well in my case), take the cover off the case, insert the wide plug from the electronics in the case to the hard drive, insert the power plug in the case to the hard drive, attach 4 screws to hold the hard drive in the case, reinstall the cover, done!
In the US, Newegg at
http://www.newegg.com and OWC at
http://www.macsales.com both have good selections of hard drives and cases.
