Depends what you are running in your system since the PSU will only draw as much as it needs to power the components in the system, it wont always be providing 1200W of power. Its unlikely that your PSU will get fully loaded, so you likely wont need a 1500W unit, but its important to factor in the efficiency of your PSU too. What PSU are you planning on getting and what are the other components in your system?
I'm planning on getting either an Antec trupower quattro; is that right; or Thermaltake toughpower
My Plan Build
Asus Rampage III Formula
2 x 6870 or 460FTW/ future upgrade to 2 x 6970
6g Kingston 1600
1TB HD/ 1 ext. HD