xterm, this might be too late, but for a gtx 580 you'll only need a 600w psu, might I recomend the:
Thermaltake W0355RE Litepower 600W $79.00 z-38
its 600w and if you check it out, you'll see that it has an efficiency of 70%
so 600w * 70%=420w and thats the minimum
now for the gtx 580 it has a power concumption of:
# System in IDLE = 175W
# System Wattage with GPU in FULL Stress = 382W
link
thats the msi gtx 580
I couldnt find one with the asus,
but heres another one with the inno3d gtx 580:
# System in IDLE = 180W
# System Wattage with GPU in FULL Stress = 413W
link
I think it should be good enough,
unless you're going to overclock it (which you dont need to...)
EDIT: I found a test with the asus gtx 580, it needs 384.2 watts overclocked,...
link