Dell D610 Laptop

I have a Dell D610 laptop, and it is continually telling me that I have low memory space. I know nothing about computer tech, but after investigating it looks like I need to buy more space, but according to the specs, the max the laptop can take is 2GB.If the motherboards already in the laptop are 2GB, what is the point of buying new 2 x 1GB? I am confused, if I replace the old ones, does that mean all the info on them is permanently  stored on the laptop and I am starting afresh with the new ones? HELP