Id go with the duel core. The quad is for people to add ram like mad. Im running it well with 2.8 ghz.

What Ive noticed is what really makes it lag is using high resolution (im using a 21 inch so it lags like mad) if You get one with like 2 gb of ram and a descent video card (normally comes with computer) just lower the resolution and it will run plenty smooth with good graphics.