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 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.