You wont be able to crossfire two 6950's with that motherboard . I know it says its crossfire x compatible it would be really limited by the second graphics card slot running at x 4.
Motherboards with the 970 chipset cant crossfire properly , except the asrock which does some magic electric trick to make that possible .
But theres not much chance you will need two graphics cards any time soon , and by the time you ever do there will be newer be a newer generation of graphics cards available to upgrade to .
Its always worked out for me that by the time I could crossfire or SLI I was always going to be better off with one newer single card .
I like this build a lot . Sensible , and a good foundation to build on when funds become available