I just purchased a 2GB DDR2-800 module for my MacBook, installed it (leaving one of the original 512MB modules installed) and when I turned the machine on, the optical drive made its usual noise, then I got nothing but the sleep light. I can boot fine with just the 2GB chip installed, and the system runs great, so my question is am I not able to mix memory modules of different bus speeds? Is this just a MacBook thing? I know with DDR the faster modules will reduce their clock speed to match the slower modules, and I thought this was the case with DDR2. So what gives?