What games are going to play or/and what application do you plan to use? Do you plan to make use of SLI (multiple video card setup)? What resolution are we talking about, if you are going between those two option for video card?
For general gaming there will be not much gain in having hyperthreading (HT) enabled as the difference between these two processor are only 100 MHz (3.4 vs 3.5 GHz@stock speed and with access to multiplier for core frequency (K version of both CPU) which you will be able to adjust yourself). Both processors will operate in most situation as quad core CPU (from OS POV it will show 8 threads for i7) as there is still very few games or application that make use of HT or even all four core from i5.
I think today it is only Crysis and BF4 that might take advantage of HT from i7 and in the near future there will possible be more games, but then one have to remember that it will be limit for how much SMT one can do in a game. (Meaning how much you can actually divide processing in a game between core or threads). There are threads here on TH that discuss this matter, so I would recommend you to search these threads yourself.
http://www.tomshardware.co.uk/forum/354727-28-does-hyperthreading-gaming
Important wiki regarding HT/processing:
http://en.wikipedia.org/wiki/Simultaneous_multithreading
http://en.wikipedia.org/wiki/Thread_%28computing%29
You can confirm this yourself, by comparing benchmarks for both processors (i5 4670k vs i7 4770k) and for the same games. The gain in processing power through HT do not always translate into a large jump in FPS and in some cases you might even be able to OC higher without HT, so a faster video card will be more efficient then to put money into HT.