The hardware to translate SATA to USB must be a part of the case you will get.
There are 3 ways to plug an external HD to your system:
Firewire
USB 2.0
eSATA
eSata is best but you need to have an eSATA connector on your PC, which generally only comes with the newest high-end notebooks. If you are determined to use it on a Desktop, you can get a controller card, though I have never seen such a thing (and you still need a free USB port to supply the card with power in some cases).
USB 2.0 is best for general backup purposes and most comon.
Firewire is best for heavy video-editing.
There are external HD cases, which will have more than one method to choose from; I recommend a case that uses USB 2.0 with a SATA 2.0 HD.
Adding to that, you should really consider if a external HD makes any practical sense and if you are even at any time going to transport it around.
I know a couple of people who have one totally exclusive for standing around on their desk;
If you don't use a notebook, where a HD upgrade isn't possible, you will most likely have a very much slower HD that is much more prone to fall of the table, or loose power/data-cable connection, so on a desktop only get one if you need mobility, because you can hardly call your data safe on one compared to a HD in the tower.
I have a 100 GB Iomega.... it's pretty small, but external hard drives are pretty expensive.
I personally have a mac, so I'm not sure if I can be much help here (assuming you have a pc) because I don't know crap about pcs. If you have a mac, I can help as I have had many problems with mine and solved them all.