well you only need a mobo that can handle a x16 PciE and x1 PciE. and btw what are you gonna use the nvidia quadro 6000 gpu for? hope its not gaming beacuse it aint worth it. it cost 5000 dolalrs for crist sakes lol. i woud rater run 6990's in 4x crossfire (if its eaven possible lol)
or if you want nvidia go for 590's in 4x sli (if possible)