Most high-end GPUs max out at 350w. A newer one on a new process will probably use less. A new CPU will usually max out at 200w, even if you're looking at overclocked intel i7 extreme editions. You will never hit 1300w with one GPU, and you will almost never get near 1300w with two.
Whoever said 700w probably took it from a website recommending a 700w PSU for a system with a 280x inside. My HD 7770's box said a 550w PSU was required, but the card itself only used ~100w max.
That all said, it's always good to have spare power. I would get a 1300w PSU if I could, because who knows when I might pick up another 2 280x's?
EDIT: Updated system specs, they were out of date.