Will my mobo work with crossfire.

Your board will work with crossfire, up to two gpus. As for powering them many come with an adapter that plugs into two molex 4 pin plugs. Also it's usually better to run one card than crossfire unless you are upgrading one you already have. When you run multiple gpu's they don't run at 100% efficiency like one would on its own. I would just buy a 6950, then you can buy something better later on and crossfire the two of those.

my psu is a 700 watt with 2 6 pin connectors and i already have a 6770 and they only use 1 6pin connector so i can get another with my free one i was pretty disappointed with the upgrade from my 4870 to 6770 except the direct 11 does make games look alot clearer but the performace of the 4870 was just as good or better . But i read on a chart from AMD that the 5000 series and up i.e 6770 6850 etc were not compatible with the 790x chipset.