For your build, a 750 would allow slight oc'ing. I would go for the Seasonic psu.
However, if you are planning to oc the cpu you might want to get something in the 850 W range.
Here is a refernce page that will give you a general idea of power requiremnets based solely on GPU(s) being used, but does not take into account any overclocking:http://www.realhardtechx.com/index_archivos/Page362.htm
This is only a guideline, not the end-all-be-all answer for your needs.
My personal choice of PSU manufacturers are Seasonic, XFX, Corsair and Antec, but that's just me.