For a single 6870 1000 watts is extremely overkill even for two 6870s 1000 watts would be overkill. The 6870 uses a maximum of 168 watts. Add in about 200 watts or so for the rest of the system and you get roughly what your system would pull. A good quality 450 watt PSU could handle your system with a 6870, though a good 500 or 550 watt PSU would give you a little more headroom if you want to overclock or get a more powerful GPU down the road.
Coolmax are not known for their good PSUs I doubt the one you linked to could actually deliver 1000 watts, look for a good quality brand like Antec, Seasonic, Corsair, XFX, or PC Power and Cooling