1) Graphics cards don't have a CMOS
2) Graphics cards don't have a BIOS
They have a VPU and onboard memory. The VPU is basically a small CPU dedicated to video processing (hence Video Processing Unit). All video card overclocking does is increase the core (VPU) speed and memory speed, so it can process more information at once. Basically just like overclocking your CPU and RAM, only on a smaller scale and much easier.
Edit: And your system will lock up if you push you video card too high. Killing a video card from overclocking is actually a pretty rare thing, so I'd worry more about those extra FPS than voiding your warranty. Heck, I've pushed my X850XT pretty damn far (and had it lock up), and it's still working.