I based usage on 6 hours idle and 2 hours load per day. I think this is a decent usage model for a gamer. Some days the PC would be off, some days lots of web browsing and some days lots of gaming. The power values for the cards were taken from Xbit labs since they provide the values for the cards by themselves. I couldn't find the numbers for the 8800 Ultra on Xbit so I used TR's numbers and Xbit's numbers from the other cards to take away the rest of TR's systems power.
The following costs are based on $0.12 per KWh
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Card Watt Idle Watt Load $/Week $/Year $/Y24h idle $/Y24h load
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Radeon HD 4850 40.7 109.9 $0.39 $20.32 $42.78 $115.53
Radeon HD 4870 65.2 130.0 $0.55 $28.52 $68.54 $136.66
Radeon HD 4870 X2 78.9 263.9 $0.84 $43.85 $82.94 $277.41
Geforce GTX 260 44.8 136.1 $0.45 $23.70 $47.09 $143.07
Geforce GTX 280 50.2 178.0 $0.55 $28.79 $52.77 $187.11
Radeon HD 4670 8.7 47.1 $0.12 $ 6.41 $ 9.15 $ 49.51
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Card Watt Idle Watt Load $/Week $/Year $/Y24h idle $/Y24h load
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Radeon HD 3850 13.5 63.1 $0.17 $ 9.08 $14.19 $ 66.33
GeForce 9600 GT 25.5 59.7 $0.23 $11.93 $26.81 $ 62.76
Radeon HD 3870 18.7 81.1 $0.23 $12.02 $19.66 $ 85.25
GeForce 8800 GT 34.6 78.3 $0.31 $15.95 $36.37 $ 82.31
GeForce 8800 GTS 512 37.8 99.0 $0.36 $18.61 $39.74 $104.07
GeForce 9800 GTX 45.3 108.1 $0.41 $21.37 $47.62 $113.63
Radeon HD 3870 X2 58.0 171.0 $0.58 $30.22 $60.97 $179.76
GeForce 8800 Ultra 75.5 144.1 $0.62 $32.46 $79.37 $151.48
GeForce 9800 GX2 69.1 181.8 $0.65 $34.09 $72.64 $191.11
Surprisingly the cards cost of ownership stays in the same order as their cost of purchase. I was really expecting the Radeon cards to get a bigger bonus from the lower idle values but the usage patterns outlined above seem to match Nvidia's low load usage better.
The formula used is Cost Per Year = Price * 365 * ((Watt Idle / 1000 * Hours at Idle) + (Watt Load / 1000 * Hours at Load))
Edit: Added the formula used, and the values for 24hr idle and 24hr load.
Edit 2: Added new cards
Edit 3: Added 4870 X2
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