A good 550W would actually be enough to run the build as is. 650W is enough for dual cards, but many like to get 750W for the extra headroom.
If the PSU you have is of decent quality and isnt too old, dont see why you would downgrade it. Having too much wattage isnt a bad thing.
Unfortunately I was moving country and could only bring along a limited amount of things, I had to leave behind my case, PSU and screen. I managed to bring along my other components in my suitcase (including the Noctua cooler still attached to the mobo...)
If 650W is enough, that'll mean a slight saving for me compared to a 750W one... excellent. Even better that the 660ti is less power-hungry, so a future upgrade won't stretch it. I'll probably go one 660ti rather than two 560ti's in SLI so 650W will be plenty?