The PSU is solid for your "HD6950" and should handle a those is Crossfire (6950's aren't power hungry). I would up it to a 650W Corsair or Antec to be safe though.
there are better cheaper MB that are not ASUS and not may not be Crossfire/SLI. the MB was chosen to meet these two criteria not because I would recommend it. its just an OK MB. using a mATX MB will save $ also.
I used a good Case that comes with 4 fans and a fan controller. you can change it to a cheaper case to bring it into your $600 budget.