I believe Sapphire provides an application for their cards, I suggest using it.
Get the application, check out the settings and start googling them so you KNOW what they do before you actually mess with the settings. But for most overclocking you'll be working with actual GPU frequency and Memory/Shader frequencies. I have heard that some of those models were locked voltage and some weren't, not absolutely sure if they are all or just some or if there's just some way you have to control the voltage (like with a certain application). From what I read on that card, you can actually run two different bios's on the chip. This could be the way to unlock the voltage if it is indeed locked. Bios's can have major effects on how the card operates. So having "two" to select from is probably like being able to have a performance mode and a normal mode.
That I couldn't tell you my friend as I've never used the 7950. Do some research on google and find out what voltages are safe to use on that card. Just remember that some cards won't do as good as others. It's a luck of the GPU draw.