If you upgrade you RAM, it would probably be best just o get a 2x2 kit (4GB) and get rid of what you currently have. Rather than trying to find a match, or mix & match RAM.
However, as others have already suggested.... and I too agree.
Adding more RAM will likely not help gaming performance.
4GB is the sweet spot for 7, but with 3Gb you should be fine. Also, with a dedicated graphics card, you will have the full use of your 3GB, so you'll gain 256MB of useable RAM.
The most noticable improvement will be getting a dedicated Graphics Card and disabling the onboard.
But you may need to look into a bigger Power Supply as well.
After you get the GPU and Power Supply upgraded, you should see a significant improvement in games.
After that, then worry about upgrading to 4GB of RAM if you still want to.
That would be my suggestion if the purpose here is for games to play better.