Is this for back up or for regular use as an additional drive?. The only reason I ask is that I have a massive one (250 GB) which I use primarily for back up, but it needs it's own power suppy, and I don't have any spare sockets so have to use an extension lead from across the room - not ideal for permanent use. However I have a 60GB drive plugged in which gets it's power direct from the USB, and it's very handy for keeping my music and film files.
Just make sure you get one with a fan, i bought one from e bay it has 300gb and cost me about �80, now i store everything on it so when i do a virus scan it does so quicker than normal