Yes a 3930k is going to be faster at encoding but only seconds faster not minutes. If it is worth it to you to shave 10-20 seconds (sometimes less) off a a task and spend $300 to do it. More power to you.
I saved the $300 and spent it on a 7950 insted. But thats just me. Many people will want to stroke their ego and boast about having a 6 core and to them i say cool go for it.
But i tell you now i dont regret one bit getting the 3820 over the 3930..
yeah I think I'm on the fence because I know the 3820 is the CPU for me, but when I built my P4 machine I decided that the dual cores were just too much money for me at the time when I needed everthing esle for the build. This system held up good for many years, but recently the single core started to hold me back, so I found 2nd gen dual core CPU for $50, and with a BIOS upgrade my board was able to handle it. The switch wasn't as giant a leap as I thought, because I was going from a single core CPU with hyper-treading, to 2 full CPUs without. At that time , I think I did the right thing because to jump up in cores you had to loose hyper-threading, though the $50 CPU overclocks 12% higher (whats really keeping it in the game).
But now you don't have to give up hyperthreading to go up in cores, I think it's the cool factor of 12 threads that has me on the fence, maybe going with the 3930K.
$289 kinda pushes me back over to the 3820 side, but the money doesn't have as much weight in my decision as it did when I was younger. Like you said you bought a 7950 with the money you saved on the CPU, so really you didn't save nothing, because we are human and we spend until we have nothing left. Right now I don't need anything else other than CPU/motherboard/memory, so only real choice for me is weather to dish out the extra, for the coolness factor( +possable longer life before needeing an upgrade)
but nothing is pushing me hard enough one way or the other, I'm just stuck on the fence swaying back and fourth. If it was something other than a CPU, I would go with the all important "which ever one looks the best!!"