Yes 650w will be fine aslong as you don't overclock the cards to be on the safe side, depending on the age of the PSU you have. They say 600w is what is required for 2 x 6870 in crossfire.
http://www.amd.com/us/products/desk...6870/Pages/amd-radeon-hd-6870-overview.aspx#3
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for AMD CrossFireX™ technology in dual mode)
Certified power supplies are recommended. Refer to
http://ati.amd.com/certifiedPSU for a list of Certified products
Minimum 1GB of system memory
Installation software requires CD-ROM drive, a keyboard, a mouse, and a display
DVD playback requires DVD drive and a DVD
Blu-ray™ playback requires Blu-ray drive and a Blu-ray disc
For an AMD CrossFireX™ system, a second AMD Radeon™ HD 6800 Series Graphics card, an AMD CrossFireX™ Ready motherboard and one AMD CrossFireX™ Bridge Interconnect cable is required