For one thing, it is most likely you are going from 256 to 512.
Also, you need to find out what type of RAM your computer takes. Then buy a 256 MB Stick of it.
When you put it in, make sure to unplug the computer, ground yourself by touching a piece of unpainted metal first off. Then make sure the clamps on the RAM slot are open and orient the RAM the correct way and just push firmly until it locks in there.
Any questions, just ask.