Broadwell will only show minimal cpu improvements over haswell. K models of broadwell will launch after locked ones, so it might take about half a year until an I7 4790k equivalent launches.
Either way, for a pure gaming system an I5 4690k + cooler will do just as well as the I7 + cooler. Even a $30 212 evo would bring the I5 to 4.4+ ghz.
Also, what do you understand under "crappy motherboards"? A $300 z97 board doesn't get noticeable more performance out of an I7 4790k at stock than a $30 h81 board does.
What motherboard you choose should depend on what features (or design) you want and need. But since $120 z97 boards in general offer all things that have real use (sli, m.2, etc.) more expensive boards tend to just add fancy buttons like clear cmos (you'd normally have to touch two metal pins), on board temperature reading + display, a killer lan chip that has yet to prove its benefits, better sound chips or even just a pcb cover for the look.