750W is what you should get for a dual card rig.
80+ Certified, 80+ Bronze, Silver, Gold, Platinum and Titanium all refer to the efficiency the PSU has when it converts AC Current (wall power) to DC Current (power the computer can use). An 80+ Certified PSU is guaranteed to be at least 80% efficient under loads between 20% and 100%. The better ratings (Bronze, Silver, etc...) refer to higher levels of efficiency.
80+ certification also specifies other things as well, such as having the rail voltages not deviating more than 5% than their rated value.