For OCing an NVidia GPU try NTune. Ntune allows you to OC and test it before applying it. so, if you go over a safe limit it will give you a "test failed" warning and not apply the settings.
DL Site:
http://www.nvidia.com/object/ntune_5.05.54.00.html
This will also allow OCing of any other NVidia parts.
There is an inherent problem with NTune. That is, that as you make changes,in this case to your video card, and save them to run as a profile or at boot-up, the NTune performance tools save the entire package and run it (meaning the CPU, GPU and memory settings).
The problem lies in areas such as variable volting to your CPU depending on load and in variable fan speed settings. Some motherboards will overide the NTune setting and some will not. If the motherboard doesn't override, you may not be getting enough voltage to the CPU when it needs it the most, causing a black or grey screen freeze. If the motherboard does try to overide NTune, it creates a conflict as the two struggle over which has control of the machine.
I had this exact issue when I flashed to a new BIOs a few weeks ago. I attempted a minor OC on my CPU and every time I booted it was back to stock settings. NTune was re-setting it. Then I disabled the NTune profile and tried to accomplish what I wanted using rules but NTune was conflicting with the system creating instability. In the end I diabled all NTune profiles, rules, etc., set everything up as I wanted in the BIOs, and now only use NTune for the temperature monitors. My system now runs smooth as silk and cool.
I visit the EVGA and NVidia boards regularily and the majority of opinion is to uninstall NTune performance tools. Since this is the only way to OC the video card, it's probably why you don't hear of too many people doing it. OCing is usually reserved for CPU's and memory as those adjustments can be made in the BIOs.
I haven't tried RivaTune so I'm not sure if Riva acts similarily or not but I know it has all the other options that are in NTune.
