If PCS don't sell the RAM as an upgrade, download CPU-Z and check the SPD tab to get the exact model of the RAM, and then see if you can buy the same from a store with a good returns policy - in case it turns out not to be compatible.
Note that there is very little performance difference for games between 8gb and 16gb RAM, and RAM is very expensive at the moment, so if you don't need to upgrade then waiting could be better.