Three last variants are of more or less the same price but if 2 X5650 2,66 GHz give a serious gain in productivity I am ready to buy it.
Then why would you consider the W5590 is the X5680 is more powerful at the same price? that eliminates one.
Now, for expandability, I recommend the X5680 because it is a single chip with higher clocks. The second or first option could perform better BUT if you get the X5680 now, you will have a less costly upgrade option by getting a second one in the future for even more performance than 2 E5640 or 2 X5650. That's my take, anyhow.
Personally, I think all are way overkill but I have little experience with HD video encoding so wait for someone else to either confirm or deny that suspicion.
premiere scales very well with core count, I'm not positive about the elements version but I don't see why not. I would go with the x5680 for the high speed and 6 HT threaded cores... If I had that kind of dough