A hard drive can have many different features.
For example, is your motherboard capable of using IDE, or SATA?
IDE are those thick ribbons usually.
Speeds can also differ with cheaper hard drives running at 5000 something RPM and average hard drives running at 7200RPM.
Threre are also hard drives capable of running at 10,000RPM, but that is overkill for what you want
Basically, the faster the platter can spin, the faster the data transfers, etc, though other things come into account, like the cache.
Size is also another issue. I suggest something along the lines of 120-250GB which will probably last you a good while in storage.
So, an IDE or SATA hard drive with at least 120GB of storage running at 7200RPM.
Makes of hard drives differ also, from Maxtor, Seagate, Western Digital to name a few.