Go and grab the little free program called 'CPU-Z' online - easy to find with any search engine.

It tells you a lot about your CPU,memory,motherboard and so on directly compacted in small tabbed windows - you don't need to open up your PC.

And it will display the size, kind and how many memory sticks you have installed.
It even lists the name and brand of your motherboard. So you could jump to the motherboards manufacturer website, select your motherboard and read the info about how much RAM can be installed on your board.