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.
^+1 agreed, personally the most important thing is having a minimum fps of at least around 50fps although I might be a little pickier than most. In a perfect world your average and minimum fps would be greater than 60
personally I would scale back the resolution to say 1650X1080, lower AA and AF settings and probably use gamer quality settings this should put you in the realm of playable crysis performance with your 6850 and yes if you overclock crysis will see nice improvements, do your research first but I've heard that there is good headroom for overclocking on a 6850, crysis is obviously a demanding game but playable to me would be a min fps of 30, however I already told you what my personal preference was in the above post^^
all this will be entirely different based on different peoples opinions. For a FPS i think a minimum of 25fps is playable, although it would be better if it was 30+fps. "So I looked up the benchmark scores for an HD 6850 and it says on crysis 1920X1200: it runs at 24.1-36.1 FPS. Are these playable FPS" < this is entirely dependant on quality settings and anti aliasing settings also. The CPU also has some effect on the FPS. If you lower all the details to High instead of Very High, the FPS will be much better.