I would recommend, for the sake of ensuring compatibility, to get rid of old RAM and populate with identical modules. A good source is found at the web site of Crucial which allows you to plug in your PC by make and model. You can find what you need and order it right there. They guarantee compatibility. I have used them many times with many PCs and never needed to return a module. Good luck.
I plan on installing new memory into my Dell Dimension L866r. Right now I have a 64 MB DIMM card installed which came from the factory. When installing memory, do I need to remove the old card and replace it with the new, or will I be able to combine the MB of the 2 separate DIMM cards when sat in different seatings?