Storage Capacity

Hard Disk Drives can be found in a variety of sizes from as little as 80GB to as much as 2TB.  As HDDs are filled with data, the drive slows down which decreases performance.  The higher the drive size, the more space for games, programs, and media files.  If you plan on using a lot of media files or installing large programs, the larger Terabyte drives are key.

Speed

A variety of levels of RPMs can be found depending upon the form factor of the HDD.  For a desktop computer the RPMs of the drive can range from 5,400 to 15,000.  The faster the RPMs the faster the performance.  An increase in the RPMs however leads to an increase in noise, though the manufacturers do their best to limit this.  Standard computing will need either the 5,400 RPM or 7,200 RPM.  Performance gaming and computing will see benefits from the 10,000RPM or 15,000 RPM.

Connector

HDDs utilize a SATA interface.  Today, they are found in two different speeds, 3.0Gb/s and 6.0Gb/s.  The newer faster connection is the 6.0Gb/s which allows for a swifter data transfer rate and speeds up performance.  This connection however must also be used on the motherboard for it to be accessible.

When building or upgrading a system, each person will have specific needs.  The above factors are important to understand when putting together a system.  Thus, through a combination of storage capacity, speed, and the type of connector used, it is possible to fine to a computer to exact specifications.