from osx 10.9.2 there is 2 nvidia gnu limit till then its 4 on yosemite its still 2 nvidia but you can use one ati for monitor
apple is afraid of 2009, 2010 and 2012 mac pros with pci expansion box to overrule new 2013 models of mac pro
I'm running 2009 mac pro flashed on 2010 with pair x5680 ...