Thank you guys!
But I just found that it only has two RAM slots, which means that I can only upgrade it to 16GB..
check this out make sure you support it though http://www.crucial.com/usa/en/CT16G3ERSLD4160B?imodule=CT16G3ERSLD4160B
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007952 600082361&IsNodeId=1&name=16GB