To reword the question:
Would you say that my electric bill would be *less* using this PSU with 1 card and ~450W worth of draw, than it would be with a near full load of 2 cards and ~600W draw?
Thanks for the replies, best answer will go out after this
Yes, not that it would be very noticeable.
You would have to have quite a few peripherals to pull the kind of loads you're talking abouthttp://www.anandtech.com/show/3809/nvidias-geforce-gtx-460-the-200-king/17