When I look at the new Intel 10 Core Xeon E7 CPU I wondered what about my Intel 980X farm.

The new fastest 10 Core Xeons come at about 4'200 US$ each. They run at 2.4 Ghz, and you cannot overclock them.

My 980X six core went at about 1'000 US$. And I run them at 4.0 Ghz and even at 4.2 Ghz routinely, and I use faster RAM (2000 MHz) when the XEON tops at 1333 Mhz.

Now let's compare

6 cores X 4.0 Ghz = 24 Ghz.
10 core X 2.4 Ghz = 24 Ghz.

Interesting isn't it. TDP are same for both at 130W.
The architecture of the E7 is more efficient. So we can suppose that in terms of overall performance the Xeon is a little better but it is not sure that this translates into significant results for the Boinc/WCG software under W7.
In terms of points/cpu$ the 980X remains unbeatable against the Xeons. You can have a 55-60k machine on a fast 980X with the right project. That translates to about 55 points/cpu$. The Xeon would be at 15/cpu$
I do not know how the 2600K overclocked would rate.

The Intel 980X was put on the market on Q1 2010. We are now on Q4 2011 and it is still the top and probably will stay so at least another year. I do not see any equivalent octo-cores coming in the near future. The 990X and 995X are pale clones with a little faster stock clocks but this does not change the fact that a good 980X can run at 4.0 Ghz on air cooling and without dramatic additional energy consumption.