Will this yield a noticable performance increase?

I would sit tight until Piledriver (next gen FX) is released! 3rd quarter was the prediction!

getting the 4170 would be a lateral move from a phenom II x4. i would OC your CPU to 3.8ghz and cross your fingers for piledriver to deliver

At some point you may want to consider a switch to Intel and yes that would mean a motherboard change. You would be inproving your FPS to a much higher level and then your options would be better.
You only have to look at this chart to see the reason why I said to change.

http://www.tomshardware.com/reviews/gaming-cpu-review-o...

This is a cpu gaming chart of the best gaming cpus from the top(best) to the bottom (worst) and you can't help but notice that the first two levels, and those are the highest ranked cpus are dominated by Intel. So much so that the first two levels only have Intel cpus.
So if your only concern is gaming and FPS then you can't help but think about getting the best.
Of course you can hope that AMD can finaly deliver a cpu that can compete with Intel , with thier upcoming Piledriver cpu. If notjing else they do come up with great names for thier cpus.

Skip the FX series, it's a waste of money if you already have a Phenom II.

I would wait for PileDriver. My guess is that performance will increase by 10% (assuming same clockspeed) compared to Phenom II / FX. That would roughly put PileDriver on the same performance level as the 1st gen Core i3/i5/i7 released back in 2009.