With a 512 and a 1GB sounds like DDr or DDR2, and while there's a good chance they will play together, it will be in single channel and by default run at the slower timings and freq of the two sticks (though you can often match them at the higher settings manually with timing and voltage adjustments. Modern Intel mobos now have what is called flesx mode where you can mix different size sticks like a 2 and a 4GB stick and would in effect run 4GB in dual channel and the odd 2GB on the 4GB stick in single channel