Buy what you can afford; 1 - 512, 2 - 512, 1 - 1024....you get the idea. Just make sure; #1- that your board supports larger memory sizes and #2- if you only replace one, that the memory speed is equal to or greater then your present ram. If you replace both then buy the fastest you can afford. For info on size and speed, you'll have to RTFM......