I have a 600W 80+ Gold PSU (effeciency 540W) so there's a bit of headroom, but not a lot.
people seem to read efficiency wrong. if you have a 1000w power supply rated at 80% efficient, it doesn't allow 800w. it allows 1000w, but it will draw 1250w from the wall. 1250w at 80% is 1000w. you get the power advertised, but the power supply will use X% more, where X is 100-efficiency rating.
another note, peak efficiency is usually around 20-75% load on the power supply.