There are a number of benefits to having an SSD drive in a Mac mini. You will especially enjoy the increased speed if you have a server that has a large database or does a number of lookups in that data base.
Luckily, SSD drives have been coming down in cost and going up with reliability. Our favorite is the Samsung 840 series that comes in sizes up to 1TB. Once you have it ordered, you can follow this simple tutorial on replacing a hard drive in a Mac mini.