Dell announced that you can no longer order a QX6800 Extreme Edition processor with its XPS 720 H2C system. The reason for this is because they can't get enough supply of QX6800 processors that can be overclocked high enough:
Note that we are still offering the QX6800 Extreme Edition on the air-cooled XPS 720 Red system running at a lower clock speed of Bin+1 or (3.2GHz).
Now for more background: even with the H2C cooling system, we found that only a small fraction of our QX6800 processors could go to Bin+3 and remain stable through our rigorous testing process. When we started taking orders for the 720 H2C we had a fixed supply of the QX6800 chips that seemed ample at the time, but once when we found out how rare the Bin+3 variety was, we started combing our supply chain for even more. Last week, our backlog forecast started to pass our best estimates for finding Bin+3 chips. It just did not make sense to take any additional orders when we couldn't tell when we would be able to fulfill them.
At this point, our engineering team is doing everything they can to get more Bin+3s out of our original supply but this is expected to be a slow process. Fortunately, a lot of the processors that don't make the Bin+3 screening support Bin+2 just fine (3.46GHz) so we are going to make this an offering very soon. In other words, soon we will re-introduce the quad-core QX6800 processor on the XPS 720 H2C at Bin +2, or 3.46GHz.