Due to having a mac pro I have to run an external power supply for a gtx 295. I have read that it needs 680 watts to run. Is that including the other computer components altogether or just by itself? I am a little confused cause i also read that it requires 285 watts of board power and I don't know what that is. So what power supply would be sufficent to run this card as a dedicated external source? 450? 750?