I upgraded from DDR3 1600 to 2133 and overall noticed a decent increase in performance, but I generally do a lot of multitasking. I may have my 2560x1600 games up on my middle monitor, chats and music up on the left monitor, and keep an eye on my system performance/temps and the progress of my bluray ripping/encoding on the right. I typically don't have less than 8-12gb of ram used at any given time. But having said that, in terms of raw performance figures I really only saw maybe 3-5fps gain at the most overall. I don't think most people are starved for memory bandwidth these days, but what really does benefit from it is integrated graphics. If you have one of AMD's Llano processors then you
absolutely benefit from having faster memory when you are using the IGP. In such a case you really are going to be starved for bandwidth.
But in terms of general system performance,
Xbit Labs did a solid write up on the effect of memory timing and speed on the performance of IVB and it would seem generally DDR3 1866 or 2133 would be the sweet spot as they balance price and performance. Anything higher would be quite expensive in a 16gb kit for example. But if you only need 4gb or 8gb you might be able to find a more reasonable price.
The impact of any upgrade is always going to be subjective. How you use it is important, and that is especially true for memory. If most of your ram sits idle and you do very little in the way of bandwidth intensive applications then there is no reason to spend a lot of money on your memory subsystem. But I would recommend at least DDR3 1600 or 1866 given today's prices.
I never felt for this PC tuning nonsense, but for just once I wanted a bit of that 
boo