well done .
yea as ap4lifetn said you can mix ram up but it all defults to the same speed i would not recomend diferent size ether depend on your slots free and your bugdet i would buy 2x 512 mb and have these in your slot 1 +3 - if your MOBO suports dual channel and move the old ramm to slot 2 + 4 that way youll have 1.5 gb ram or you could buy 2 x 1 gb and have 2.5 depends on budget