The best and usually easiest way to overclock is to use the brand's utility, which IMO is almost always better than the OverDrive.
You didn't mention your brand name, but if it's branded Gigabyte/ASUS/Sapphire/HIS, use their utility, it'll be much nicer than OverDrive.
Moving on, you should probably start with the memory clock, as you don't have to worry about the power running to it as much when upping this.
Try ~50MHz ups at a time (though your first jump should easily be ~100-150MHz no problem), and then run a game for a bit. Make sure everything's stable.
You could also download a benchmark program and just run it.
Furmark/
Heaven are two of my personal favorites.
Then, you'll start overclocking the 'clock' settings on the GPU, again you should be moving up at ~50MHz steps, but it'll most likely only run near ~800 on your card.
Here's a guide that I think would help you as well (especially because he also references Furmark, which I tend to favor, and he explains the test/scores as well).
Hope this helps!