Using a simple board like this will put a second SID chip at $DE00 and add 3 voices to your system, with suitable software and a selection of SID files you can play music on your system like never experienced before (your mileage may vary)! For everyday use you can use it in pseudo stereo-mode by having each SID play the same thing simultaniously.
As with anything dealing with SID chips you should know what you are doing, this board does not deal with the specific power requirements used between SID types and putting the wrong thing in there will certainly kill the chip you value the most or both of them!