ok, to see if you have any free memory slots, download belarc advisor.
run belarc and under the "installed modules" section in memory, it should tell you if you have any ram slots free.
the most you'll have free is obviously one, although it's quite likely you'll have no free slots.
if you have one free slot then everything is good, you'll be able to buy another 512MB stick no problems at all.
if you currently have no free slots you'll have to throw away your current ram and start over again.
run belarc and tell us what it's saying