Hey i have this psu Corsair HX520w ATX12V v2.2 and EPS12V 2.91 520W Power Supply and it states it has 3 18A 12v rails, im looking into getting a 7900gs vid card, and minimum requirements state a 12v rail with 20A reccomended 28.... are they refering to PSU's with single 12v rails or will my PSU not have enough power? thanks