OK I HAD NO IDEA THIS GOT THAT LONG.
Now that I think of it, it might be. It allows for a lot more headroom and after all, PSU's decrease in wattage over time. With A higher wattage, you have a lot more "watts unused" (don't know how to say it), but it will keep efficiency higher at load. I still feel it is too much. It's the recommendation because not all PSU's have the better efficiency ratings and may mean that 750W PSUs will probably not get to even 650 watts with bad PSUs. But with a 650 watt Gold rated PSU you are getting around 590-610 watts of usable power. With a 750 watt bronze PSU you are only going to get as far as 690 watts or so due to heat output.
I'm sorry I made that long, but IMO I would probably get the 650 watt PSU (Enough power, Gold-rated, fully modular, cheaper)but because 750 watts will last longer at only a bit higher, I guess you could. But you are just missing out on the gold-rating and modular cables. But who cares. I'm just saying that you can save 10-20 dollars-ish and get a more ideal PSU OR your can opt to get an 850watt PSU for a bit more so you can crossfire the 280x.
The 280x specifically is an odd card. There is a huge variance on power use between models with some chewing up 300w and sometimes higher if over clocked with the right psu hooked to it. (highest I saw benched was at 319w with an oc on a matrix platinum from Asus, which I have, followed by the sapphire toxic at 308w. some cards with much lower settings can run on a 500w psu though and use no more than 175w on default settings... These things range in base clock from 825mhz to 1180mhz with major vram differences as well within the 280x model line)