Quoted:
I've always been told to buy memory in exact pairs. So you should replace the 1 GB with 2 X 2GBs at least.
It depends upon the manufacturer.
Rambus memory is designed to work in pairs.
Others can be done either way.
What you want to do is look at the Current memory modules in your PC. Write down the specifications.
Example:
DDR3-1066 PC3-8500 CL7 512x64
Several months ago I replaced those with
DDR3-1066 PC3-8500 CL7 1024x64
If there are open slots all you have to do is get as many more memory cards as you do open slots.
If all the slots have been taken you get to upgrade the memory like I did.
Be Sure the new memory speed is exactly as the older chips. If they aren't you'll never get all the memory you are paying for.
After the new memory has been added make sure the BIOS is updated as well. Many systems don't have software that can take full measure of any additional memory.
CompTIA A+ Certified.