Hello... this page for your MB say you can put Two more Sticks of memory... http://www.asus.com/Motherboards/M4A79T_Deluxe/specifications/
Adding 4gb or more is possible with your MB... Try to locate similar models/brands like you have now... DDR3 all look the same but some communicate in a different manner, Make sure if you get a differnt brand/model , that they are compatable with your MB.