Well lets put it this way: If you're looking to buy that PSU and a pair of 6850s I would defiantly get a larger PSU than that one. If you own the PSU and a 6850 already then adding a 2nd card will work fine enough, the PSU just won't have as much head room for extras, such as multiple disk drives, CD/DVD drives and overclocking.
EDIT for more detail: The big thing is that yes, it is recomended to have a larger PSU watt wise, but that PSU is pushing out 40amps on the 12v rail. It is recomended to have 35amps for 6850s in crossfire. Generaly lower watt PSUs won't have that kind of amperage on the 12v rail.