This might sound like a very nooby question but I've to ask it anyway! Upgrading my dads stone-age PC, and want to add additional RAM. He has 1x512Mb DDR (400Mhz) in there right now. If I add a 1Gb DDR (400Mhz) stick to increase to 1.5gb, will this cause issues? I seem to have a bell rigning in the back of my head telling me to match memory sticks like for like. I.e. I'll either need to add another 512Mb stick, or take the 512Mb out and add one, or two 1Gb sticks... can someone clarify please.