i would recommend 750watt or higher (make sure its a good brand named PSU).
if you dont have enough power your computer may not start, or the GPU drivers will underclock your graphics card but its best to not put your system under that type of stress
give the 600watt ago but i think you might be pushing it
it will work on the 600W just fine when the power supply is new; however, when the capacitors in the supply begins to age, you're going to have a higher and higher probability of a problem. If I were you, I'd spend the extra $20 and get a 750W supply (I recommend corsair)... after all, you're spending like $320 or so just for your gfx cards.