Assuming a 100% load utilization and generous allowance for additional devices (six fans, an extra PCI card, overclock, four USB devices, and four ram DIMMs), you are looking at a MAX draw of 650w.
That would be simultaneously pushing your CPU and GPU to their limits, which would never happen outside of a linpack and GPU test at the same time.
However, if you add a second GTX 570 (SLI), your power consuming jumps to 850w, which is probably the line of thinking of people suggesting a PSU over 1000w.
However, if you have no plans on going SLI, a 750w PSU will serve you fine.
Just remember, total wattage is not the most crucial factor when picking out a PSU, but total voltage available on the +12V, +5V, and +3.3V rails.