Although temperatures in the 70's for a GPU are indeed normal, to know if the card is working good you require do to more research. It depends on tendencies rather than the value itself.
I read in your post "After one hour or so playing..."
Does this mean that the card starts at a lower temperature and then gradually raises? Because if that's the case then there's something wrong with your cooling. It's not necessary a matter of good airflow, you can have that, and still have a card behaving weird.
I had a card which turned out to be busted, always hanging at around 70C, It gave me issues with some games. I RMA'd it, had the same model and the card now hangs at around 60C
So, if the cards starts at 70C(on load), and maintains itself and 70C(while on load). You're fine, if there's a gradual increase something is not quite right. The card, the airflow, or the environment. Then again, if it stays at 70C-72C and it no longer increases after that, it's not really something to worry at. As long as your games work fine.
Just in case I'd recommend trying it with high resource consuming games that the card claims to support. I knew there was something wrong with my card when Bioshock Infinite and BF3 BSOD while the card spec said that those games were supported.