Depends on the motherboard, but if you go to crucial.com, they have an app that can analyse your PC and advise you whether an upgrade is possible, and if so, the best combination of chips.
http://eu.crucial.com/eur/en
Having said that, I doubt if increasing the RAM will speed up your machine for playing games. You probably would be better of upgrading the video card (most games rely on the GPU to do all the calculations required for positioning and rendering the graphics)