Depends on you. You could just get two 6870s and be done with it. You can also get one 6970 now, and get another one a little bit latter. If you know that a single card is going to have a bit of trouble at the settings you want to play at then go with the two 6870s. If one 6970 is going to be fine for now then get one 6970 and add a second one latter on.