Using the PSU calculator linked below, I come up with 598 watts w/ a single 275 and 787 with twin cards (920 CPU, OC'd to 3.75,1 SATA, 1 DVD, 5 USB, 1 firewire, 10% capacitor aging).http://www.antec.outervision.com/PSUEngine
At the least, I'd get an 850 watter that gets a 10.0 performance rating at jonnyguru.com .... in alphabetical order
XFX Black Edition 850
* requires compatible case
The XFX is going for $90 on newegg at the moment, after $20 promo code and $40 MIR.