Considering your primary uses and native res, I don't recommend a CPU upgrade. Going to an E8400, even at 4GHz, won't be worth the money IMO. Your games are far more GPU limited at 1920x1080, so I don't think you'd actually notice much improvement in that area. Encoding would be faster, but not worth $150-200 faster.
If you opted for a quad instead (Q9550 or Q9400 for example), the video encoding would definitely be faster. However, unless you do a lot of that it probably wouldn't be worth while. And as with the E8400, you wouldn't notice much / anything in the way of gaming improvements (except for the few games that actually benefit from a quad).
I also don't think you're going to get much more out of your E6550. 3.4GHz is a very respectable OC for a 65nm C2D. If you pushed it another 100 or 200MHz I think that's about the max based on a safe vcore.
My advice is if you want more fps for gaming, go with a GPU upgrade instead. Other than a GPU, the next logical upgrade for you is going to be to i5/i7... and that's not a $200 upgrade.