This HTPC I'm typing on is a similar PC, with an old (but fantastic at the time) 5770 ATI card. Comparing it to my gaming rigs, which are socket 2011 and 2011v3 platforms (similar performance to the 4790k), it's quite a large jump, but much of that is based on the GPU as well.
A 4790k platform is right up there at the top of the gaming CPU systems you can build. I've said this often before though, that various gaming magazines and sights have tested all the CPUs currently out there from the 2500k up to the socket 2011v3 stuff like the 5820, 5930k, and 5960x and so forth - with the same video card there isn't a whole lot of difference, ie it's mostly down to the GPU if you have a decent CPU issued in the last couple/few years.
I would say though that since the i7 900 series is a fair bit older than any of the newer i5/i7 platforms from the last couple years, this may stretch out to a higher value, and be well worth it. Of course, the GPU is still going to be the most important feature in terms of gaming performance. What GPU are you using now, and do you plan on upgrading it? 970 cards are under 300$ if you search about, or get a used/newish one, and that IMO would be an important thing to upgrade as well as your MB and CPU. Of course along with that likely comes a PSU upgrade, and so on, so it can add up quickly. Just upgrading your CPU alone, assuming your MB will take it, and considering you won't be able to get that much faster of a CPU on that current MB anyhow = not worth it. However upgrading the MB, Cpu, and GPU(depending on what you have now) will be quite the jump in performance, especially going from your current setup to a 4790k with a decent GPU.
edit - A shame Midway is banned from the BBS, this type of inquiry would have him spasming all over the forums, with news of a potential Kazaa super FPH return based on upgrading. Ha ha.