It could depend on the limitations of your motherboard. Do you know what the make and model is?
Or it could be incompatibilities with the ram. have you got the code numbers for both sticks and also are they both ouble sided or isone or both single sided?
Or it could be that your computer just accepts 32M memory and the 512Mb dimm could be 64M. If that is the case, it will show just half of the total ammount.