Odd, X2 3800+ and an 8800GTS 640 here and I have no issues. Stuttering and lag may also be slow/fragmented hard drives too.
Keep in mind that making games use multiple cores requires a shift and more complicated code than a monolithic game. Most game engines are written in the form of an action loop - process user input, process AI, process sound, process graphics, update graphics, rinse, repeat. Add in physics and such as requested. Once you move to multithreaded, and inherently multi-core code, you start complicating this process. The loop is used because it creates a finite period of time the code has to run, and you know what performance to expect as a programmer. Multithreaded/core use means breaking this loop, and having what amounts to essentially separate programs handling bits and pieces of the loop. You may be thinking "well just have the loop call these different programs and you use multiple cores utilized!" There's the issue of timing. If you make it multithreaded in this way, you gain nothing because you have to put in waits and checks to see if the other thread has processed it's job. (The other threads can be on other cores). What really needs to be done is shift the paradigm how the game engine is written. There are a few things that can more naturally be broken out of the loop, since they don't depend nearly as much on the graphics and user: AI for example can run full time, causing the world that ISN'T on the screen to be updated all the time (and the world that is on the screen too). But for example, sound is usually tied directly to what is on your screen, and synchronized with the graphics. It's much harder to sync sound via a separate thread.
There are many other issues that makes programming multicore harder - resource allocation in the program for example. Let's just say unless the developer specifically targets multiple processors with their product, don't expect multicore CPU's to ever do much until we fundamentally wrap our brains around multiprocessor designs on a daily basis (and no more single core mainstream CPU's are still in existance).
