Thumb of rule when you increase the voltage on certain devices, ie: CPU/MEM.
Make sure you got sufficient cooling. Increasing it, you know the risk involved. Obviously increasing voltage may increase the stability of the overclock. If necessary your not getting a stable speed on a overclock, move up the voltage till it is stable. Use common sense. Like your CPU is default to ie: 1.65, move it to 1.66, etc... Not like a huge leap like 1.65 to 1.71 or whatever.
As for your videocard, try get a fan for it if your going to overclock it. It is a lil more safer than without it.
I usually do this when I overclock for memory. Increase the FSB incremently to a highest speed to a stable state. After that, if I want to go further stable, increase the voltage a notch. Then increase till satisification.
Some memory can take the extra voltage. I know mine does fine at 2.8.