AFAIK Mav is right, so get a card which matches it for speeds, and it also means that if you overclock them they are likely to get to similar levels so you wont be too limited by the weaker card.
AFAIK Mav is right, so get a card which matches it for speeds, and it also means that if you overclock them they are likely to get to similar levels so you wont be too limited by the weaker card.