70 degrees is just fine for your card. As outlined by the GeForce website, your cards thermal limit is 98c, at which point it will start to permanently damage your card. DO NOT take this to mean you should overclock until you get your card running at 95c, just because you can. The cooler your card is, and the further it is from it's thermal limit, the better.
As for the temp limit, I'm fairly sure it has the power to downclock your card should it ever reach the specified temperature limit. If I were you, I'd put it up to 90c, as 80c is just fine for your card to run at.