Often users do not know or forget that a PSU will degrade over time. You can estimate that a PSU will loose up to 10% output every year. The better quality device you have will lower that rate. Even so, having the extra overhead to support future HDD's and memory upgrades is nice to have. The cost for more is not that much so very worth it in my opinion.
the quality of the components in the PSU factors larger than what wattage it is.
there is little reason that a quality PSU @550 watts cannot run any single graphics card solution.
It would have to be a POS or kept in very hot temperatures for it's capacity to degrade by 10% a year.
Obviously if someone wants to keep upgrade options open then it's better to buy a higher capacity PSU to start off with because you spend less overall.
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.