That is not a legal DDR Configuration. You have to have two 512MB and two 256MB modules. With DDR, you need to buy memory modules in pairs. It is also important to have the modules in the correct RAM slots. Your system or motherboard manual should tell you how to properly configure your memory. For now, take out your 256MB Module and just use the two 512MB Modules. Make sure they are in the correct slots. By the way, your original configuration of 768MB was also incorrect.