Do you need to change the mobo to do this? If so, might not be worth it, unless you are like me and like to support AMD.
If you do NOT need to change the mobo, you will see a gain, moreso if you OC the 8350 to 4.8 or so. You will need a good aftermarket cpu cooler to do that.
However if you leave the video card the same, your gaming fps will not jump a whole lot.
As part of an upgrade path to R280x/R290/R290x/770/780, it would make sense to swap in the 8350.
If you are only doing a single upgrade to this pc, spend the money on a higher end video card, get a good cooler for the 955, and OC.
That would give you the most bang for your buck.