Easiest upgrade to your current PC? An SSD drive

The single easiest upgrade you can make to your current desktop? Adding an SSD drive. Nothing will help your computer speed up than putting in a drive that is usually 5 times faster than what is in your machine right now.

The strategy I like to use is to put in a 60GB drive in for the operating system, then have an additional traditional hard drive in the machine for storing your data (audio, video, pictures, etc.)

OCZ is a great brand (Intel is another strong leader). Here's my favorite for around $100: The OCZ Vertex 2. Get it for around $100-$120. Put it in, reinstall Windows (Windows 7 can be optimized for it... search for AHCI and switch your bios from IDE to AHCI for the drive *before* you install windows).