That really depends on the type of 280x. In my experience Sapphire Toxic needs more power than the rest. If it's not a Toxic you should count 230-270W per card (undervolted between 1080mV - 1150mV). For Toxic 300W to be sure. I'd go for 3x 500W (650W) PSU (should be able to get gold cert. for $100 each). If you dont have mobo with molex, you should get some powered risers to reduce the risk of burning your mobo.