It depends.  First you have to understand how important is the hard disk’s performance in relation to the other components of a computer.  Remember that compared to your processor and memory, the hard drive functions mechanically.  Thus, it’s response time is relatively much slower than these other components.  But since most processes take place in the CPU, very often the hard drive is neglected as a contributory performance factor.  To best understand the overall impact of the hard drive’s performance, I suggest you read these PCGuide articles about Hard Disk Performance and the Importance of Hard Disk Performance.

To help you decide whether you need an upgrade or not, you have to identify whether most of your processes are I/O bound – when a lot of read and write instructions to the hard drive is required.  If so, then upgrading your hard drive is a very good option to improve your system’s performance.