Intel i7 4790K vs i5 4690K

For gaming: no. You would not see an appreciable performance gain in any known game. If you have extra money and "need" more performance, go for a better GPU. Or, if you didn't already, an SSD (not for gaming performance, but for loading times and general PC experience).

For gaming, they are pretty much the same. Get i5 and a better GPU.

i7 just looks better, but you will get same performance with the i5, not noticeable difference with the i7 or even not any difference, because "the cool look" of an i7 doesn't make games run better, i7 is better than i5 for other things but not for gaming.

Are you going to overclock? Will you need the igpu in it at any point? If no to both get Xeon.

The biggest difference between equivalent i5 and i7 CPUs is the i7 supports hyperthreading, and hyperthreading usually doesn't improve gaming performance. If you're only going to play games, get the i5, and if you're not planning to overclock it, save some money and get the non K version. I'm still running a first gen i5 750 (2.66Ghz stock). When I was breaking it in, I was able to overclock it to 4.2GHz and pass 8 hours of a stress test, although it got a little hot for my taste, but no game would put that much stress on it. I put it back to the stock 2.66GHz to install Windows, games and other programs. At stock speeds it ran everything I had maxed out, so I left it stock figuring I'd overclock it later rather than upgrade. I'm still running stock clock speed on it and only now am I starting to run into games that I can't run maxed out. I think most of those are GPU limited though as I'm using a GTX 660.

if you plan on using multiple monitors, yes i7

i5

Get Xeon!

Currently an i5 will do, but if DX12 is to take advantage of multiple cores and threads like they say then to future proof for a long while an i7. I'd also hold out for Broadwell or Skylark if you can.