Yea, a beefy CPU can break 300w, more if overclocked, add 350w for the card and some for the rest and you are probably well into 700w+
I think some (reviewers) have shown a 650w psu can cope but that would be a lesser setup and you would be on the edge, so it best be a bloody good psu and you may be using SATA/Molex adaptors *shudder*. Would you really put a £700 cpu on a £60 psu?
When I looked I didn't see much price difference, and the RM 850 comes in at a reasonable price (relative). I actually got an RMx 850 in the end.