The ASrock 970 Pro3 can support:
-- 4 DDR3 DIMM cards
-- DDR3 2100(OC)/1866(OC)*/1800(OC)*/1600(OC)*/1333/1066/800 non-ECC, un-buffered memory (These are the different RAM frequency/ speeds your board supports)
-- Max. capacity of system memory: 64GB
Looks like you currently have one RAM card of 8GB Corsair DDR3 1333 RAM installed. If you wanted to build more into your system the most cost effective solution would be to order another exact copy of the RAM card you currently have, upgrading you to 16GB total. 16GB of RAM is usually as much RAM as you'll ever need.
If you are looking for places to upgrade your rig in the future you can keep in mind that your motherboard supports much faster RAM that the type you currently have. You are now using 1333, while the motherboard can support frequencies up to 2100.