Your Motherboard determines the maximum amount you can install (due to RAM slots available) and Your CPU and Operating System determine how much of that RAM can be utilized. A 32 Bit OS running a 64 bit processor can only utilize a max of 4 GB, regardless of the amount you have installed. a 64 Bit OS running with a 64 Bit processer can utilize 18 GB of RAM or more. This has to do with the way the OS is coded to utilize the pathways of the CPU.