If you're looking to buy a Hard Disk Case (sometimes also referred to as a "Caddy" or "Enclosure") then you first need to know if you've got an IDE Hard Disk or one of the newer SATA (Serial-ATA) hard disks. Its pretty simple to check as the connectors on the end are very different - see images below:
Generally SATA drives are faster than IDE but you wont see much of a difference until you get into the higher RPM (10,000) drives or RAID setups. Generally IDE drives max out at 133MBps while SATA is capable of 150MBps and some go as fast as 300MBps with more speed in sight.
Its also worth noting that if your are using an IDE (or ATA) Hard Drive with an Enclosure/Caddy then the jumper settings on the Hard Drive need to be set to "Master" and NOT to Cable Select or Slave.
The following are some of our IDE/ATA Cases:
Pluscom 3.5" IDE USB 2.0 Enclosure - Black
2.5" HDD Disk Case + One Touch Backup
Mobile Disk 2.5" IDE Hard Drive Enclosure
For SATA HArd Drives choose from the following:
Probox SATA HDD to USB 2.0 Interface
3.5" Mobile Disk USB 2.0 - SATA HDD Enclosure
AcomData E5 SATA Case + SATA PCI Card