what watt psu should i have for 4 cards or for 2 cards?
depends on the other hardware being used. a pure mining rig will have a single core CPU drawing 45w or less, a single stick of RAM, and no hard drive(BAMT on a thumb drive). You want about 75w for all this. If you use a quad-core 95w+ cpu, multiple sticks of RAM, and a mechanical spinup hard drive, you want more like 200w for all that, to be safe. Add another 200w per 5850 being used, 235 or so watts to be safe.
for 4 cards in a pure mining rig, use a 950w+ supply.
for 4 cards in a normal rig, use a 1200w+ supply.
for 2 cards in a pure mining rig, use a 550w+ supply.
for 2 cards in a normal rig, use a 700w+ supply.
5850 requires at a bare minimum 18amps on the 12volt rail to run stable, but you want more than that. try to use a power supply, in all of the above setups, that uses a single 12 volt rail. I'd suggest using Corsair, seem to be the best stability/power/$ ratio that i've used. g'luck.
