Your system has 4 slots for memory and is capable of going to a max of 32GB. This being said you do have a few options for upgrading. You can go to 16GB by adding an 8GB kit (two 4GB modules). You may also add a single 8GB module and go to 16GB that way. Or, if you decide to go to 24GB instead, we do offer a 16GB kit that you could add to the existing 8GB.
A complete listing of upgrades can be found here. Please let us know if you have any further questions about these upgrades.