Memory upgrade from 512 to 1,024 problem

Well, it seems that both RAM sockets worked previously with the 256MB DIMMs; remove the 512MB DIMMs, and install the two 256MB DIMMs again, then boot the system to see if both of the smaller DIMMs are recognized. If BOTH are recognized, then one of the 512MB DIMMs is defective; test each one individually to see which one is not working. If only ONE of the 256MB DIMMs is recognized, then you might have a small piece of dirt or debris in one of the DIMM sockets; it is also possible (though not as likely) that one of the DIMM sockets has recently become damaged while you were changing the DIMMs... Shut the system down, REMOVE THE BATTERY AND POWER CORD, then examine each DIMM socket to see if you can find any dirt or debris blocking the DIMM from making proper contact in the DIMM socket. Also be sure to check the DIMMs for damage, or the presence of a foreign substance on the gold contacts. You can use a pencil eraser to GENTLY clean the gold contacts on the DIMMs; this will help to remove any light surface oxidation that could be causing this type of problem. Post here again to let us know what you've learned...

Are both of the ram PC2100? Because this mother board only supports PC2100 memory modules.