16x/4x crossfire won't make a difference at 'normal' resolutions (1920x1080), but when you go up higher or use Eyefinity there is a measurable difference.
I say measurable because it's 2-3FPS difference.
I had read an article on it, but it seems to have disappeared.
Go with the ASUS P8Z68-V LX, it's a very solid board (not as easy to overclock cause you have to work with CPU VCore offsets, instead of just setting a voltage, but this is the "correct" way of overclocking the processor).
I would up the PSU a bit, get it modular if you can...
Also, get a tube of Arctic Silver 5 thermal paste.
Other than that, very nice build.