I just went to crucial.com and says my pc can handle 2048 of memory..inside?

currently i have 256 memory cards in both slots making 512mb... and right now its on 448mb due 64mb used for onboard graphics.. i got a game .. it requires 512mb of mem... can i buy a 512mb memory card ddr2 replace the 256 card.. so then 512mb card and a 256 mb card makes it 768mb.. will that work ?