Part of the memory is used by the graphics card. You probably won't see a performance difference between 2gb and 4gb unless you're running a 64 bit os. In fact, I don't think 32-bit os'es support 4gb of ram. I think they will only show 3gb. You don't really need a matching 2GB stick as long as they're both ddr2 and at least ddr2 667. I have a couple 2gb sticks for sale right now if you're interested. P.M. me about it.