Usually the drivers
in the gpu package are outdated,so is better to download and install the most recent software\driver
,catalyst from here, AMD Graphics Drivers & Software
Your AMD 7950 is powerful enough to handle almost anything.
So you dont need to use the overdrive or any overclocking.
If you just increase by 30-50 in gpu and ram clocks then you dont need to change any voltage.
With 30-50 you will not see any improvement anyway.
If you want to add 100 and more clocks then you will need to adjust voltages for stability issues and adjusting voltages is not a good idea especially if you are not an advanced user .
My opinion don touch anything,ok almost anything
,you can adjust freely the fan control