for 7970, 1150MHz core is a sweet spot for 24/7 OC..it's good if you stay at this clock first and do heavy gaming for a week or two. If no black screen,artifacts, driver resert or game crash during that period then you're good to try even higher though I dont feel a need if you go a little bit higher.

Your card might be able to pushed up to 1200MHz+ but temps will rise proportionally and that's not a good idea for the long run. so if I were you, I'd stay at 1150MHz for heavy gaming and stock speed for normal games which dont require max gpu power.

Temp at 74max is good..to me below 80'C is considered fine enough. you can use valley,heaven,3dmark and some other heavy games benchmark like metro LL, metro 2033, AvP etc also to test OC stability..