To display FPS in Diablo 3 you press Ctrl +R.
I have a GTX 660 and when a lot of stuff happens on screen, FPS
tends to plummet seriously. Wine still has performance issues with
this game. Also, don't replace the 560 ti with a 750 or 950. They're
not even in the same league. The 960 is not a future proof GPU.
Newer games will want to use more than 2GB VRAM. If you can, get a
GTX 970 or wait for the new generation of GPUs in 2016. I'm trying
to "limp" along until then myself :)
No real issues myself in D3 here. Now that wine likes loading it up without some massive magic voodoo ritual it runs quite nicely.
For the record I started off on an i7-960 + 12GB Ram and am now on an i5-4460 + 32GB ram. Each use my older yet stable and fast enough 560ti 2Gb Nvidia. This game is quite playable at all times. Perhaps there is a GPU setting or whatnot that I do not have enabled/disabled and you do that is changing the performance. I had used the guides from back at launch to get it going and frankly, other than reverting from the custom launcher, no other changes have been made (well OS updates -- RHEL 6 -> 7 ... but I don't think that matters).
That said, I do agree with the holding off. The price/performance curve isn't there yet for the current gen cards. Barely double the performance for $700 is not my idea of a deal. I recall the days of triple performance every two years at the $200 price point. Not saying that we are going to see that ever again, but to go from that to anything else just isnt cost effective.