ummm if you have say 2gb ram and your thinking of adding old rubbish ram like 512mb sticks DONT BOTHER - not worth the hassle, and will more then likely result in worse performance because of the additional sticks to address and worse timings/speed - i see people do it all the time, its stupid

also if your cpu fsb or imc required 667mhz ram then your old 533mhz sticks will not work (E6x50, E8xxx, Q8xxx, Q9xxx etc)