From what the MSI website says of your board its not to picky on ram. if you already have 512 then two more 512 cards should hold over for everything but intense gaming
below is a link to 1gb of ram for 32 dollars, it is within spec of your board.
http://www.newegg.com/Product/...?Item=N82E16820141215
set them as dimm 0 and dimm 1, and if that isnt freindly try dimm 0, and dimm 2.