I vote for the monitor.
For gaming, a I7-4790K with an overclock will not be significantly faster. Few games can use more than 2-3 threads so the I7 hyperthreads will not be very useful.
A second GTX970 will win you fps benchmarks, but gameplay is generally better with a single good card.
You may encounter stuttering or screen tearing.
Also some games do not support dual gpu anyway.
If you want a graphics upgrade, sell the GTX970 in favor of a GTX1070 or GTX1080.
You may be able to find a GTX980ti at a good price, it is comparable in performance to the GTX1070.
A second monitor is most useful. You can keep email performance monitors or whatever on your old monitor while gaming
A side monitor does not impact gaming.
Yes, 1440P does have more pixels to push. But the higher dot density lets you reduce the AA which is one of the more performance hurting knobs.