i bought a 750GB Seagate Barracuda 7200.10  it's a SATA disc. The serial number is 3QD0AH7V. 
then i bought a hard disk enclosure from ebay. when you buy enclosure make sure that you buy the right size and right type. common sizes are 3.5" and 2.5". they are widths of hard drives. and types are IDE and SATA, SATAs are more common these days. the enclosure comes with everything you need - USB cable, power cord, screws, and even a screw driver, instrucitons etc.
all you have to do is screw your drive inside the enclosure and you have an external HD.
my HD is 3.5". if you can get 2.5" HD, it'll save you a few bucks on enclosures because 2.5" enclosures are cheaper than those for 3.5" drives. one thing i don't like is the power cord - it's a bit rigid, and kind of awkward looking but it does the job.
here are the links to HD specifications and enclosures respectively: link for enclosure shows IDE drive enclosure, SATA enclosure looks similar.
 http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_7200_10.pdf
http://cgi.ebay.com/3-5-USB-2-0-IDE-Hard-Disk-Drive-HD-HDD-Enclosure-Case-/200456303590?pt=PCC_Drives_Storage_Internal&hash=item2eac2073e6#ht_2734wt_1204

thanks for your wilingness to help. will let you know if needed.