No 650w would be really really pushing the limits for Crossfire 6950's. 750w+ is recommended.
2GB 6950 over 1GB 560Ti will not make a whole lot of difference in multi monitor gaming as people might think. TOMs hardware did an article recently comparing 1GB and 2GB cards over multiple screen sizes and the results were remarkably similar in that the 2GB added very little if any at all noticeable advantage. Especially running duel cards your going to be packing at least 2GB Video RAM anyway.