Actually most high-end applications such as Autodesk's 3D Studio Max or Maya utilize four cores very efficiently and the difference in rendering time is very noticeable. A simple search for quad core benchmarks will show you what does and does not utilize the extra cores.
Here is a nice benchmark list that you can select several choices from. Hopefully the link I posted will narrow down the choices between the E6700 and QX6700, which are essentially identical in architecture, clock speed, bus speed, and cache size, with the only difference being that the former has two cores where the latter has four. You'll notice that the 3DS Max benchmark will show the quad core doing the rendering in almost half the time, where as if you were to look at a benchmark such as AVG's virus scan or iTune's encoding, you'd see the results are identical.