Take a screenshot of the motherboard tab in CPU-Z, I am not sure about your motherboard model at all.
If CPU-Z shows DDR3 RAM on the Memory tab I would say the maximum is 8 to 16GB (more likely 8, it will show you the amount you currently have and the sticks used if you make your way through the Slot list at the SPD tab, may you take screenshots of that aswell?).
CPU-Z: