Actually 650W is a good go for SLI/Xfire systems while single-gpu practically always stay below 450W. A 5850/dualcore setup will probably run perfectly well with a 300W unit.
Recommendations of suppliers on the other hand have been over the top at all times. Nvidias 7800GS for example got a 400W (or even 450W?) recommendation with at least 20A on the 12V rails. I ran mine perfectly well with a 250W/13A unit. Then I overclocked and overvolted it to a consumption rougly in the league of a 7900GTX - and continued to run it perfectly well. I even made a testrun with the much more powerful HD3850AGP an this power supply - 100% stable. (though it was dying for weaks before anyway and this test actually was its last boot up
Some say, this is to accomandate for potential power hungry CPUs, but my rig had an overclocked Pentium 4 Extreme Edition at this time, so I attribute the high recommendations only to the low quality of some PSUs on the market. As long as suppliers put 600W-signs on units, that could hardly deliver 400W continously, the manufacturers are forced to recommend 650+W units in case a freakish Quadfather/Skulltrail owner comes along and wants to sue them for blowing up his PSU.