Re: Optiplex 3011 AIO CPU upgrade
I did some looking and so far, the fastest I've seen is the i5-3470S, which is about 50% faster than what you have.
Of course the big difference is going to be the 4 physical cores, which will increase overall speed and multitasking a lot more than just the pure performance jump. The only issue is if there are two different cooling solutions for dual-core vs. quad-core - the newer the systems, the more likely they are equivalent, but 3-series is right on that edge.
You might want to check that out, comparing heatsink-fans from the two different configs (use teardowns, reviews, images, Dell parts, etc.) just to make sure they are the same.