as what t_blake07 has said, you will need to know what type of RAM your new comp supports, what type of RAM is already installed, and what type of RAM you are installing (DDR, DDR2, memory speed, etc.)
take note, if you add, for example, DDR2 400 to your computer that already has DDR2 800 installed, the DDR2 800 will be limited to 400. you will have more RAM, but it will be slower.