Are you on vista or XP? If XP, it's safe to assume you are on 32 bit. On Vista go Start --> Computer --> System propeties and there will be a bit saying system type, and that will say whether its 32 or 64 bit. As for RAM, whichever really, depends if you think you will every upgrade again, as two 512 sticks will be a fair bit cheaper. But so long as you're running pairs, you wont notice a difference between 4*512 or 2*1024. All you need to make sure is you get RAM with a speed supported by your mobo, an jusding by your specs I'd say that it's would be up to 800mhz. If you can find out what mobo it is, can give you a definate answer.