From what you have told me, it sounds to me like a lack of video processing power.  If you ar running in integrated Graphics card (built in to the motherboard), you will continue to have issues no matter how much ram you add to the system.  RAM is not going to be your solution.  What you need to do is a little research, or reply back to this and I will do it for you.  You need to find out if your computer is capable of accepting a PCI-Express or AGP video card. 

Computer gaming in todays world requires the use of a Video card for two reasons: 1. it takes the load off your processor. 2. It has its own memory.  Integrated graphics cards nad low end agp/PCI-e graphics cards don't have the power necissary to render extreme graphics.  Integrated, in particular has an issue because it is sharing memory from your system RAM, leaving you little ram to work with, as well as being short on the shared RAM.  WHere as a video card has its own...