In fact a "quality 450 Watt would be plenty.
Anything OVER is gravy. JUST remember do NOT skimp on QUALITY of PSU
Ref: total system power for i7-965/X58 OCed to 3.75
Measured power consumption R7750
..System in IDLE = 155W
..System Wattage with GPU in FULL Stress = 188W
..Difference (GPU load) = 33W
..Add average IDLE wattage ~10W
..Subjective obtained GPU power consumption = ~ 43 Watts
Even adding a 2nd 7770 for xfire sytem power would be below 250 W
My own system, max power consumption at wall = 350W
..i5-2500k w/OC to 4.2 Watts ( more than user's system
..ATI 6870 draws more than 7770
..16 gigs ram, DDR3-1600 CL 7 @ 1.60 Volts, again more than users'
..2 SSDs, one HDD, and one blu-ray writer.
Yes PSUs do degrade over time, but it is NOT a linear function. Might ge a smal bump in the first year, llevel off for two to 3 years, then start degrading small at first and rising until EOL
Eff. drops off at both ends of the spectrum, below 20% and above 80%.
Ideally you want a PSU that when at Idle you are above the 20% (ie In this case Less than 650 Watts. and 80 % for Max ( 250/.8 = 312 W).
Based on min max of 312 and 650 any Quality PSU between 450 and 550 Watts would be an excellent choice.