ASUS makes the Z97 boards the best. The i5 4690K is the farthest you can go in 4th gen i5 chips.
Also if you plan on gaming on 2-3 monitors the GPU will be bottle necked so you might as well look into upgrading the GPU. Maybe to a 780 or under $200 a 960.
The i5-4690K can run at 3.5GHz but it will run LOWER if not required and can Turbo up to 3.9GHz if needed as well (assuming you use default or XMP settings).
I recommend just using the "XMP" profile which you may already be at because it won't disable any Intel power management (choosing your own settings can) which would probably choose closer to 4.1 or 4.2GHz.
So again, the default is actually 3.9GHz max, or closer to 4.2GHz with XMP and you should simply use the XMP profile.
(Even though you can overclock an i5-4690K as high as 4.8GHz I don't see the point. It would rarely help and just adds to the heat and fan noise.)
Any processor on Intel that ends with K or X can be overclocked and has unlocked memory multipliers. On the 4690K 3.5 GHz is achieve by BCLK at 100 MHZ and memory multiplier at 35. So if you wanna overclock just bump the memory multiplier maybe to 36-39. So for example memory multiplier of 37 and BCLK of 100 MHz you would achieve 3.7 GHz and so. When you overclock a bit farther you might need to tweak the voltages or leave them on Auto.