Overclocking doesn't always void warranties. Afterall companies like ASUS and MSI offer overclocking utilities, and even if it does void the warranty, other than BIOS flashing your card they won't even know.
So, OP, my personal opinion is that there's no reason to buy a factory overclocked card. The only real benefit is if it has a much better or maybe quieter cooler, or in some cases like ASUS TOP GPUs, they are the top binned components so you can get a good overclock out of them yourself.
But paying JUST for the factory OC itself, IMO, is not worth it. Not unless you're absolutely against tinkering with your hardware, like sadams04 said, "install it and forget it".