Yeah I'd recommend 750W, maybe even 850 if you have lots of HDDs and peripherals.
I concur. If you're running a very basic system, a solid 650W can handle it provided it's got a good 12V rail. But if you want a beefier rig, an extra $20 - $30 for a 750W and peace of mind is worthwhile.
650W is fine for a single card, but it's pushing it for SLI. I've been running two 560 Ti's in SLI with a 2600K, four HDD's, an SSD, two DVD drives, a fan controller and 6.3 million fans for six plus months on a Corsair 750TX with absolutely zero problems (even with Prime 95 and Furmark running at the same time for 60+ minutes).
I agree with K1114; 6950s use less power than GTX 560 Tis on maximum load, despite having the higher TDP rating.
The problem with using two of either of those cards with the Corsair 650TXV2 is that it doesn't have enough PCIe connectors.
I would like to know where you are buying from so that I can make the most suitable recommendation for a unit which you should use.
The Asus P8Z68-V LX motherboard has an x16/x4 PCIe lane split, which is sub-optimal for pairing cards of the calibre of 6950s/GTX 560 Tis.
It will be fine however because your PSU is of high quality. Most people will recommend against it however that is just to take into account discrepancies and the ideal range of load to keep a PSU in is 50% to 60% under continuous load to keep it running at peak efficiency.