the 4670k is 3.4 Ghz and doesn't have hyperthreading.
the 4670k is 3.5 Ghz and has hyperthreading.
HT is very valuable in mulithreaded apps like video editing, rendering, CAD etc.... it has scarce any value in gaming.
Whether the extra 0.1 Ghz is of any value to you at roughly a $100 premium is something only you can decide.
I can say that in my experience, with HT enabled, on average, I have been able OC the non-HT CPUs slightly higher than ones with HT. OTOH, if I turn off the HT on the HT CPUs in BIOS, then I have been able to OC the HT capable ones higher. This makes sense as with HT on ya have twice as many threads which generates more heat which makes it harder to OC.
In short the OC'd CPU ranking would be:
OC'd 3770K w/ HT off > OC'd 3570K > OC'd 3770K w/ HT on
OC'd 2600K w/ HT off > OC'd 2500K > OC'd 2600K w/ HT on
Too small a lot of experience as yet but I assume it will follow that:
OC'd 4770K w/ HT off > OC'd 4670K > OC'd 4770K w/ HT on
But again, the stock speed advantage is just 3% ... are you willing to pay $100 for that 3%. If ya budget limited that that could mean the difference between a 760 and a 770, save the money and get the 4670k .... if you are not budget restricted, then get the 4770k....if your comfy with the marginal speed advantage it gets you.
Actually, I have spent over 3k and built several systems. The I7 4770k is worth the money it is only 100 bucks more and my computer is noticeably faster. Bottom line, the people that say the processor is only a little better do not know what they are talking about. As I have had both processors.
The i5-4670K will be perfect for gaming, but if cost is no issue to you, an i7-4770K will be slightly better (minimal), but is not worth the extra money.
The i7-4770K is much noticeably better in video editing/rendering.