People having superficial knowledge about computer hardware might think that Computer RAM is the ultimate memory component and having tonnes of it in the system would ensure that everything has been taken care of. However the truth is completely on a different plane. First of all Computer Memory Components in computers in the form of RAM is not the ultimate things as, RAM is volatile in nature, and its utility is limited only to enhance the operation of the system when it is functional.
Other than that once the system is shut down all the data that is in computers RAM gets deleted from its memory. The second misconception that having loads of RAM in the system would enhance its capacity is again baseless. The amount of RAM that a system can have depends largely on the motherboard of the system. If the motherboard states that computer can handle 512MB of DDR RAM, then it is advisable to stick to that only, as more memory than that would be of no use as the system would only run at the capacity that it can handle.
So if one installed 1GB DDR3 RAM in place of 512MB DDR2 RAM that would simply entail that you are indulging in wasteful expenditure as your PC would not be able to outperform what is stated in its configurations. Therefore it is always advisable to take advice about your memory purchases from people that are knowledgeable about the hardware aspect of computers as it would only facilitate your purchase all along.