*sigh*. For non-threaded apps, a Duo and a Quad running at the same speed should execute in about the same amount of time. The issue (and misunderstanding) is that for a given price, you have the choice between a faster duo or a slower quad.
I say again, at the same clock speed, at best, the Duo is as fast as a quad. If the Duo is clocked faster, then it depends on the app you are using.
I've noticed most newer games use quads much more efficently. At this point, only budget rigs should run Duo's...