To check your memory go here and select "Scan my system" (you dont have to buy it from them, you can buy it anywhere)
http://www.crucial.com/
It should tell you how much memory you have, how much you can fit (maximum), and how many "slots" you have.
If at the moment you have two slots with 500Mb in each then you will have to remove both of them and replace them with two new 1Gb.
Replacing memory is usually easy. There may be a small "door" on the underneath of the laptop and undoing the screw should show you the memory. It is simple to take out the old memory and put in the new.
It should explain this in the documentation that came with your laptop.
Upating the memory on a laptop is one of the easiest things you can do (assuming you buy the correct memory)