A 1200W PSU seems excessive, but you might want one for overhead. I haven't pushed the Habanero up to 750GH/s yet, but the one test hashing right now at
http://eligius.st/~wizkid057/newstats/userstats.php/1CAqwaYmNn6SZbk4G3e9cXxsJ3tJVfGuzt is pulling 54.1A at 11.92V, or 645W. That last 10% might really push the power usage if you need lots of voltage, but I'd be surprised if you couldn't get away with a good 1000W supply.
Edit: I just went and measured it again as that seemed a little low, and the number is 59.1A. Sorry for the confusion.