You can NOT use 4790K on X99 platform.
X99 is a 2011-v3 socket while Z97 is an 1150 socket.
Intel i7 4770K & i7 4790K can be used on Z87 / Z97.
Intel i7 5820K, i7 5930K & 5960X is used on X99.
X99 platform also has more PCI-E lanes. The CPU's have either 40 PCI-Express 3.0 lanes (5960X and the 5930K) or 28 lanes (5820K) available.
X99 platform uses DDR4 RAM which is no compatible with Z77/Z87/Z97.
Z97 uses DDR3 RAM.
X99 platform uses quad channel memory while Z97 uses dual channel memory.
If you plan on running 3 more GPUs then X99 is the way to go.
If you plan on using 1 - 2 GPUs then Z97 is the way to go without throwing money away.