Microdirect is a great site, as is ebuyer.
I bought some stuff from overclockers UK that was a bit cheaper, and even though everythings fine, the boxes were a bit more scuffed than I have ever got using Micro Direct or Ebuyer.
But, still a reputable store.
650 watts would be fine
As long as it has enough 12v power lines, as it says to put a different 12v line on each connection, and theres 2.