If the 480 consumes an average 100 more Watts of electrcity per day it will take 20 years for the 6870 and 570 to make up the savings.
Bit confused by this statement, Power consumption already includes time; it won't be using up 100W more 'per day', it is just '100W more' fullstop. If you run it for one hour, it will use 0.1kWh extra. So for your numbers to make sense you'd have to be only using the gfx card around 1hr a day.
I'm not saying you are wrong to downplay the power consumption factor, but your numbers don't really stack up considering how long most enthusiasts have their PCs on for.
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