Generally 30FPS or higher is considered playable, but 60fps is the sweet-spot as it looks buttery-smooth.
Overclocking will increase performance, but it depends how much you overclock. For Crysis, you might want to consider a combination of CPU and GPU overclocks to get the desired FPS without overheating either one of the components, and Crysis uses both CPU and GPU quite well. Your cooling matters of course, so make sure you have decent cooling to compensate for the added heat. I can't stress enough, though, that you do your research before overclocking anything. Every piece inside your computer will have an impact on your ability to overclock, from the PSU to the motherboard construction (how it's laid out, quality of components including capacitors and chokes, quality of heatsinks, what chipset you have, BIOS), your CPU model and stepping, RAM, and of course your case and how well that is laid out and how the airflow is set up.
Your GPU, being most likely designed after a reference card, will be largely standard. Many programs can be downloaded from the manufacturer to help you overclock the GPU. There are also a few 3rd party apps you can try. But again, do your research as not to burn anything out. How much can you overclock? Not sure. It depends on a lot of factors including heat, cooling, voltage, and YOUR card. Not every card of the same make and model (or CPU) will overclock the same as another. Slight differences in the internals of the components will give you either better or worse results as someone else with the exact same part.