This thread is about Antec 550 NeoHE and don't wish to still it. But this can provide some guidance. I have a 550 NeoHE too but was thinking of swapping out for a better wattage.
This is from Corsair directly (for Corsair 620w PSU):
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In our internal testing, we've identified that at full load, an 8800GTX consumes around 160-170W of power from the +12V rail. This is about 14A.
Two of them total to 28A.
The rest of the hardware is dependent upon your configuration, but an Intel Quad-Core processor (QX6700) overclocked to 3.3 GHz on an eVGA 680i motherboard draws about 11A from the +12V rail. So now we're at 39A.
Add a couple hard drives at 1A each, you're between 40-45A on the +12V rail at your ambient case temperature (which is most frequently between 35C and 40C) and our HX620 can handle 50A on the +12V at 50C, even more down at 35C and 40C.
If you have some huge number of hard drives, like more than 6, you might want to look into a higher-wattage supply. Keep in mind, it's not running at those ratings at idle.
At idle, the G80 draws about 7A, or 84W of power. The processor draws 4A, or 48W of power. The same system above that draws ~45A at full load will probably draw less than 20A at idle.