2 options for you
1. Buy a regular external harddisk(for example maxtor external harddisk)
2. Buy a normal hardisk + external casing.
I always go for the 2nd option as it is cheaper compared to the regular external harddisk. There are many external casing available in the market. If you are going to use the external harddisk continuously, then you have to buy a casing with built in fan.
USB or Firewire : Its up to you. But i find USB is more common compared to firewire. Or you can buy a combo which has both.
For harddisk you can go for either Western Digital, segate, hitachi.