650w is good, but it depends on what power supply you get.
I recommend the EVGA G2 or Corsair RMx.
Evga has great support
a good 550W would also be enough.
If for a new build it's usually recommended not to go straight for crossfire/sli, it's an inefficient system and not every game receives a boost anyway, you're better off getting a more expensive single card which will, no question about it, give you increased performance across a wider range of titles.