MEMORY UPGRADE. Computer had two 256 mb ddr pc2100 sticks for a total of 512mb of ram.Installed

Question

Computer had two 256 mb ddr pc2100 sticks for a total of 512mb of ram.Installed two 512 mb ddr pc2100 sticks computer starts up but monitor does not turn from amber to green light.Can install one 256mb and one 512mb everything works fine and it shows 768 mb of ram. All I want is to increase it to 1gb of ram if it is possible.