I have a story I like to tell people who think there's no advantage to quad-core rigs.
Those people usually use computers only for gaming, so they have no idea what a "core" actually does, other than it costs money. So that is why I did this like I did it, and not use a 3D rendering or some video encoding example.
So anyway, this friend comes to my place right after I bought a new beast with a Q6600. I was playing Sins at the time. Being one of those playstation-instant-gratification-people, he wasn't impressed with Sins, so he asked me to "show him something awesome". So I alt+tabbed from Sins(mind you, when you do this, Sins continues to run in the background), and I load up Crysis, which runs pretty well with all settings on High(no AA, ~32 FPS). After he is gobsmacked by the graphical beauty of two minutes of Crysis, the speakers announce that one of my planets is under siege, so I alt+tab back to Sins to deal with the problem. This is when he actually noticed that Sins was still running in the background. After being amazed at the sheer power of the rig, my DVD burner opened to eject the freshly burned DVD.
No, you're not going to feel the 2nd, 3rd or 4th core in Sins. But you will feel it if you like multitasking.