As with any overclocking, it's a risk, but the Overdrive limits are quite low, so you're quite safe as long as:
Do n't touch the voltages, even small increases can vield large temperature increases.
Keep an eye on temperatures, I like GPU-Z for this, it's free, does not need an install and records about 5 minuets of activity if it's left running in the background.
Stress test the overclock: Many like to use Furmark as a stress tester but Unigine Heaven or a later version of 3d Mark are also very stressful.
It's generally accepted that 80C is fairly hot and anything over 85 is pushing things a little too much!
You can set a manual fan speed in the Overdrive window, just do n't forget to click 'accept' to make the change stick!
Or you can just use the 'Autotune' feature, it seems to take forever, though, just watching those coloured lines.