Because you are paying $450 per card for a 1-5% boost in performance:
Review here:
http://www.guru3d.com/articles_pages/asus_geforce_gtx_780_strix_6_gb_graphics_card_review,18.html
Also, you'll probably want to stick to 3 way SLI. 4 way is horribly unoptimized and buggy as shit. It works out okay for benchmarking, but is really terrible for gaming.
To save a bit on the motherboard, check out the ASUS Z97-WS. Alternatively, you could look at just about any X79 motherboard and go with a 4820K. Socket 2011, as far as gaming goes, is pretty much designed for your purposes. It has 42 PCI-E lanes, whereas the consumer socket (1155/1150) only has 16 PCI-E lanes. The motherboards like the ASUS-WS or MVII Extreme use PLX chips to multiply PCI-E lanes. While it gets the job done, it does introduce a smidgen bit of latency that you can avoid by going X79.
*edit*
And as per above, if sound is an issue to you, I definitely would suggest looking at the possibility of watercooling that setup. You'll go from a fairly noisy machine to something that is as quiet as a single GPU PC at idle.