Hi there,
Unless you need the features of a X79 board (large number of PCIE lanes, quad channel memory) then an i7 3820 will not be an upgrade from a i7 2600K.
If you look
here, the performance at stock speeds is very similar and when both are overclocked the gap disappears.
As for overclocking, the 3820 doesn't have an unlocked multiplier - but unlike LGA1155 boards the BCLK can be safely increased on the X79 boards. Therefore, you should be able to achieve similar overclocking results with an i7 3820 as you do with a i7 2600K - though the 3820 is a more power-hungry CPU, so it will need a better cooling system to achieve the same temperatures at the same clockspeed.
As for PCIE 3.0 -
looking at the ~£400 HD 7970's performance on PCIE gen3 x16 vs gen3 x8 (gen2 x16), there doesn't seem to be any performance difference yet, even with the most powerful cards.
Since your P67 board
already supports the upcoming Ivy Bridge 22nm CPUs (with a BIOS update) then if you really want a CPU upgrade then the best route is to wait until the ivy bridge release (expected to be the end of April) and get a CPU like the i7 3770K then.
That said, (depending on overclocking performance of the Ivy Bridge) the performance jump going from an i7 2600K to a 3770K isn't expected to be massive (5-15% dependent on application) - so you should ask yourself whether your current CPU isn't up to the job and needs the money spent on an upgrade.
Most i7 2600K CPUs should be able to hit 4.4-4.5GHz overclocks (even with relatively low-end coolers)- maybe it would be worth stepping up your overclock a bit if you would like a CPU performance boost.
