Make sure you get the same type of RAM that is already in your system, you can't use just anything that sounds good.
  It is best to check what is in your system now and try to find an exact match as far as the type and speed, this information should either be on a sticker somewhere on your PC or you may have to look at your current RAM module which should have a sticker on it displaying it's properties.
  You can normally add larger capacity modules, for example a 256MB stick and it should work fine with your current 128MB stick as long as everything else is the same.
  If you try to add RAM that is not even close to what your machine was designed for, it won't work at all.
  If you add a module rated at a higher speed than your current RAM, it will only run as fast as your slowest module.