Buy more RAM. Your system will allocate all the RAM necessary for your graphics card, but only if there is any available. Given your system only has 512MB installed, there probably isn't any left over to allocate to the graphics card.
Look up your system's specifications to find out the maximum amount of RAM it supports and what type (PC-DIMM, DDR-266, DDR2-800, DDR3-1333, etc...) of RAM. Report back with this information and we may be able to assist you better.