As long as you get a USB 2.0 hard drive or a FireWire hard drive, you'll be set.
The first thing you'll want to do when connecting the new external hard drive is to re-format the drive in HFS+ format (Mac OS Extended), as most external drives come formatted as FAT32 (which is good for cross-platform use of the drive, but if you intend only to use the drive with the Mac, format as HFS+). You can use Disk Utility located in the /Applications/Utilities folder to format the drive.