According to AMD a good quality 500 watt power supply with about 40 amps on the +12 volt rail(s) and two 6-pin PCI-e video card power cables will be sufficient for a gaming rig with a 6870 video card and overcvlocking. The recommendation is valid for both configurations you mentioned.
Your question about power consumption is diificult to answer. It depends on what you are doing with your pc. Power consumption varies quite a bit between idle and something that might require a full load. The two power hogs in a system are the cpu and gpu. Other components use very little power.
About Cooler Master - They've had their ups and downs. They've had good psu's, average psu's, and bad psu's.
I maintain a list of psu recommendations. The list is up to date and based on web sites that have the proper equipment and people to conduct rigorous testing and in-depth component analysis. The list contains links to competent reviews: