Ram problem/question

Hi all, another newbie here. I have just purchased a 1gb double sided ram stick. My plan was to drop that into my pc with the 512mb single sided stick, thus creating a total of 1.5gb. NOPE! I get a total of 1gb!! My qustion, is this because of mixing the single sided with the double sided? I have swapped the ram around several times and cleaned out the slots but to no avail. Everest home edition recognizes both the 1gb and the 512mb sticks, but the scanner thing on the crucial website says that I have two 512mb's installed. The motherboard (ms-7061) supports upto 2gb ram. Please help.:cry