MSI G41-P33 COMBO

Hi, I am using an old <Expletive removed> pc which has msi p33 combo motherboard. Currently, I am using 6 gb of ram (1333). 4gb+2gb. Just want to add another 4gb stick. But the problem is, I tried twinmos and Adata 4gig kit with same bus speed. But my luck is not good. After that I found...