If you plan to buy an QX6850, don't expect any speed increases in most of your applications. Even the most popular codecs such as XviD and DivX are limited to two threads, which means no speed increase at all. You can encode and still have two cores free to do other things, like gaming while you are waiting for the video to finish. Some software vendors will support more than two threads, such as rendering software or Photoshop CS3.
Normal users will not benefit from a Quad core. For the price of one Core 2 Extreme you can buy a second system with a Core 2 Duo, but of course two computers will use more power then one, even with a Core 2 Extreme installed. For small companies or semi-pros who are involved in 3D rendering the QX6850 may be a nice addition compared to the costs of a Xeon-based workstation.
To be fair, this is the fastest Quad core processor around and if you want the best and you don't care about the money, get one of these. We will try to overclock it as well in part two.