To clear up what Lovin' said, how many physical ram sticks your computer can handle depends on the motherboard, some can only take 2, some can take 8 or more.
The best thing is to consult your computer manual or open the computer and have a look inside.
Also remember that in a 32-bit OS (Operating System) you can only have 3.5GB ram, no matter how many sticks you put in (You could put in 4 sticks of 2GB each, aka. 8GB, and it would still only recognize 3.5GB), to make your computer be able to access more you need a 64-bit OS.