With DDR the frequencies can be different (although not recommended) and they will both run at the same speed as the slowest installed. It's also critical that you don't mix different types of memory, for example, Crucial & Kingston as this leads to a lot of hardware blue screen issues (but if you get it lucky you'll get away with it).
Due to software restrictions imposed in the 32bit Windows you can only get around 3GB of memory and the rest doesn't get registered. The work around would be a 64 bit operating upgrade which will remove the cap.