8GB ram is enofgh for you as per your system configurations but 500GB ssd is a good option but that wont help any increase in FPS you can buy 240GB and save some money?
Adding the SSD later when ya have more moola is easier than upgrading from 8 to 18 GB.... Id do RAM 1st and add the SSD later. Remember SSD only helps what fits on the SSD ... with games hitting 40 - 95GB these days, that might mean a small portion of your game library fits on the SSD.
8GB Of ram is enough for gaming however I would still recommend going to 16GB first then the SSD. It's been shown that the more ram you have the less likely you'll need to write to your pagefile which is commonly stored on your primary drive.