BCCHardware has this head-to-head comparison of the latest Intel offerings. Here's an excerpt:

Intel claims that the Core 2 Duo is "The World's Greatest Dual Core Processor", and I believe that they are right. What can be better than "the world's best" dual core? Quad core my friend - quad core. The idea behind a good dual-core processor is that when the machine is doing one intensive task, it still has a processor left to handle other tasks. For most people dual core is ample, but for some extreme users today, quad core may offer some benefits. We loaded up our rig with a bunch of applications and ran WinRAR, Quicktime, Adobe Premier and scanned for Virus at the same time and both systems performed quite well. With so many intensive applications running, the QX6700 setup ran smoother however and felt less laggy when trying to open applications like Office 2007 or Firefox.

In reality, most people will not be running multiple instances of video encoding simultaneously at the same time they are compressing files and doing other tasks. The Quad Core QX6700 makes this possible, but it's overkill for many users.

Unless you are going for pure bragging rights a quad core processor really is ahead of it's time. While there are a few real world applications that take advantage of this processors multi-threaded power, most do not. In theoretical tests, the quad core runs away with the competition, but in reality it's a give and take. Now that the QX6700 is upon us, more software development companies will start taking advantage of multiple cores and then the processors will excel. If you are planning on spending $1300 on a processor that you will trade off in 6 months, I'd have to recommend the X6800 over the QX6700 at this time. Overall performance with the X6800 is marginally better than the QX6700, but I'm sure that will change in time.