First make sure all your cables are well put away so they don't block the air intake of your GPU or other parts. Make sure all the case in and outlets ( don't now if that's the proper word ) are dustfree, maybe if there is room for it place another casefan.
Maybe there is an other way but you can download a oc program like msi afterburner and you can adjust the fan speed curve there, it will work harder at lower temps so it maybe will stay below 75-80. Still 80-85 under full load is quite normal i believe for a 580, max is 100C. ( but i wouldn't wanne go that high )