Around 675, iirc their peak efficiency is above 90%. So ~610 watt at the meter is what I'd consider safe.
D3, I think you might have your numbers backwards. Power at the wall is always higher than the power supplied. Power supplied is what the PSU is rated for.
Plus, peak efficiency is typically around 50-60% load. And I don't think there's any 750W Corsair PSU with that kind of efficiency.
Since he didn't actually say what model it is, I'm going to assume a TX750 V2.
Quality review here:
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=230So at the power draw he's at, he's MAYBE getting 85% efficiency. So 790W draw at the wall is about 670W supplied by the power supply, or a little less than 90% of the power supply's maximum capability. It's not the most optimal situation, but he's well within the capability of the PSU. If it was a lesser brand, I'd be concerned, but I wouldn't be too worried about the Corsair unit.