You shouldn't buy based off rating, you should buy based on the quality of the unit. While the higher the efficiency, the more power you'll save, don't pursue a Titanium or Platinum rated PSU. If you are talking absolute best of the best, the Corsair AX1600i takes the cake. AX1500 and EVGA T2 follow behind, but all are upwards of $200+. A more reasonably priced PSU is the Bitfenix Whisper M and the Straight Power 11. If you are in the EU or UK, which you are, I'll suggest to you the Straight Power 11 from be quiet!. 650W would be plenty for any combination of CPU and GPU you plan on going with. Just ignore the argument that everyone else is having above, purchase the SP11 and call it a day.