DON'T buy a premade external .
get a regular 120 . you can get the for about 70$ or less after rebate . and get yourself a usb/firewire enclosure for about 30$ so you have it for 100$ or less ....
find a hard at like office depot/max , staples , best buy , circuit city blah blah blah . like
this one find yourself a coupon for 20 off 100 for staples and you got it for 70$ after rebate .
and get one of
these it will take up to a 300gb hd . it is 35$
so for 105 you got the external you want that YOU can upgrade to 300gb if needed ...