This is my first time overclocking a video card so I know nothing about adjusting the voltages and by how much a video card can overclocked compared to a CPU.
I went to CCC ATI Overdrive and increased the GPU clock and Memory Clock while ATI Tool 3d view is running and after increasing the clocks some more (I can't remember the numbers) my computer freezes and my screen displayed a blue background (not a BSOD just a blue background) and had to turn off the computer via the power button. This happened again after a couple of hours, but this time while benchmarking with resident evil 5 on highest settings and setting the GPU clock: 935 mhz / Memory Clock: 1270 Mhz with ATI Overdrive. The video card temerature was about 63-66 celsius during the benchmark.
Why did that happed, do I need to increase voltage?
You'd probably have to increase voltage, using something like MSI Afterburner, but I'd run your card at slightly faster than stock first to see if crashes at all, then run it at around 915MHz/1240MHz, see if it crashes, if it doesn't, go up in a few MHz intervals from there on. If it does crash, go to 900/1220, and go from there.