If you are gaming, video editing, multitasking then fx 8320 is a pretty good choice considering you will not need to get a new mobo. 4670k will right now be better in gaming over fx 8320. But that can't be said in the future. In the future with amd 8 core optimised games, it will be hard to tell gaming performance differnce between fx 8320 oc and 4670k.
So I suggest get fx 8320 and also get a nice cooler which will be able to oc fx 8320 till 4-4.2 ghz at good temps.
If you don't plan on oc, doesnt' matter, just leave the turbo boost on and it will hit 4 ghz whenver possible while gaming.
the 4670k is going to stomp all over an 8320 for gaming, having said that unless your running a pretty high end graphics card you arent going to see the difference you will be gpu bottlenecked in most games. a few games really dont work well with amd cpu's though and they would run much better on the intel rig, skyrim, mmo's and COD games come to mind.
Oh, and if you have no plan to overclock, save your money on getting the non-K edition. the K editions are only really for overclockers, they have unlocked multiplier. Maybe even just get a slightly lower end i5. If you do end up deciding to stay with amd, spend a little extra and get an 8350 since your not overclocking.
What graphics card do you have?
The cost of $110 vs +300 is highly convincing. I'd just get a new CPU if you're happy with the motherboard. The gap between Core i5 and 8320 is not big. Single threaded performance is somewhat faster on Core i5. With a highly multithreaded game, the 8320 may be able to catch up to or be close. The best value of course, is when you also overclock the 8320.