I think 2 6950s would run on 750W. That one will handle one just fine, but yes, you'll need more for Xfire.
You might as well get 8gb RAM. It's only about $25 more.
That mobo's a little expensive, but whatever you want. You can get a plenty decent one for $140 after MIR: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Monitor Resolution: 1024x768 and you will spend money for CrossFire?...biggest waste ever..If you don't want to change your monitor with a good one (1920x1080, S-IPS Panel, good response time) even a lower card like 6850-6870 can do the job for now!