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 128GB drive in for the operating system, partition it 50 gig for the operating system, and the remainder for files. If you need more space and have a desktop, pop in a 2 or more terabyte hard drive in the machine for storing your data (audio, video, pictures, etc.)
Samsung is a great brand. The Samsung 840 (or older 830) are excellent choices.