How much power does a 5830 need? I did some research and saw that each one takes about 380W at full load? Is that true?
Most likely no. using 2*5850 with 550w corsair PSU, both ~10% OC'd (mem clock at 300 though), 2*1tb hdd, i3 530.
Barely any fans (1 chassi fan, 2 fans for gpu, 1 for cpu, all stock), no cd/dvd drive.
However i don't recommend 550w for 2*5850, i'm actually a little bit scared something might fuck up.
tl;dr: they most likely run at max 150w at full load, let's say 200w to really be safe:
Calculation to determine how many watts you should get to be really really be safe: 1.10*(175*x cards)+150=whatever
(*1.10 because PSU is not 100% efficient)
Having about 50w less than what this calculates shouldn't be a problem though.