Yes, it voids the warranty and can easily destroy permanently your graphic card. If you don't know much about it, don't do it.
If you do want to try it, go by 25-50 MHz increments, then do a burn-in test for stability. Never touch the voltage, unless you don't mind potentially destroying the card.
Even with a stable overclock, you will reduce the lifespan of the card. Eventually it will start to produce artifacts (green lines and weird errors), and die.
Overclocking is a bit like racing cars. It's fun but don't be surprised when the engine blows up.