You will be fine with 750watts.
Check out this 4890CF power consumption test: http://www.techpowerup.com/reviews/ATI/HD_4890_CrossFire/23.html
They pushed their 920 to 3.8G and peak power was 420W for the whole system
However I would recommend buying a modular psu instead of a standard one. They're a bit more expensive, but you'll love them when building your pc and trying to arrange everything as neat as possible.
Cooler Master should be good quality psu's, but you might want to take a look at Corsair. I have one for 2 years now and it has not let me down yet.