The ASUS site does not list that mobo- did it come as OEM from a system manufacturer? Need more system info, but any of the major brands ( Kingston, Corsair, Samsung, etc., should be OK)
hold on, so you want to have a total of 1 gb of ram? you would have to buy 2 sticks of ram of 512mb cuz you probably only have 2 slots and those 2 slots are filled with 2 X 256mb sticks
if you do get 2 sticks of 512 try to get them in a pair so you can work them in dual channel