The speed of your RAM will hold you back as much as the speed of the onboard memory of the video card. Almost no games (none, probably) actually use all of even a 2.0 GHz processor. The rest of the computer is too slow to keep up, so it inevitably has to wait, thus wasting cycles. Getting a second CPU really doesn't do much for games... Rendering 3D images in Wings 3d and such will definitely see a boost, though it might not be worth the cost of a new motherboard, case (most desktop cases won't fit a server motherboard too well) and CPU.
