Yup exactly. It's like 16 lane highway vs a 40 lane highway and each of your monster truck nVid's use 8 lanes each. They won't all fit at the same time on a 16 lane highway. Huge bottle neck!
Good choice on RAM. You're obviously going for speed/near real time rendering so fast mem is a must.
Nice GPU's! Can't say for sure about noise level but one tip to keep noise and heat down is to get a large airy case with a large fans. They move more air at slower speeds and therefore are much quieter.
Thats an 7" fan on top set to low speed. Two 4" fans on the front combined with the 7" on top all set to low speed and they move a lot of air and its very quiet. Xeons also have a nice feature when it comes to heat and that is they have a built in switch that shuts down if they get too hot so you never have to worry about cooking your CPU.
You'd need a bigger case than this Antec 900 tho. I just noticed how close the PS is to that last PCI slot. Plus it looks like on both the ASRock and the Gigabyte that some of the header pins will get covered up by the 4th GPU. Just some extra things to consider. Very tricky cramming 4 GPU's into a case.