->just watch videos over youtube to find out the best stable overclocked configuration card of the same version.just overclock by 20-30 mhz step by step and if no problem occurs then keep overclocking(only upto a limit).i think 850/1400(core/memory) mhz would be fine enough for 6850 to run stable.
->cooler master usp100 is a good looking case.and gladiator 600 is still nice for it's price.
it does not matter which company you go with for a case.
just look for a case which is nicely build(mainly made of steel),have cable management holes(very necessary),good airflow,and can fit upto 10" cards easily.
thanks buddy,
and also tell how are how are cabinets with built in power supply , i mean i can get one for about 5-6k ,are they good enough?