Any external will work fine.
I would look for one that has Firewire and USB.
Your Mac has Firewire which yields more reliability. You'll want the USB for PC use, unless your PC has Firewire which is not usually a stock item on PC's.
Have one of these,
http://www.amazon.com/Memorex-32023288-Double-Layer-Recorder-External/dp/tech-data/B0007OWASE/ref=de_a_smtd/102-9732298-4601769
Firewire, USB, Dual Layer, reasonable price, name brand, have used on Macs and PC's with no issues.