There seems to be alot of disparrancy between enthousiasts about just how valuable "energy efficient" really is. This post will tell you just how much money you will save by getting a more greener card, how much extra money SLI is gonna cost in electricity bill and all this sorted between EU and US.
The price of 1kWh in Austin, Texas: €0.0914/$0,122. (Gexa Energy 1/2th year fixed)
The price of 1kWh in the Netherlands: €0,2385/$0.318. (Eneco Energie)
To keep things simple, im going to assume that that PCs are used idle fulltime for 9 months. This is equivalent to 2 years 9 hrs a day, 3 years 6 hrs a day and 4 years 4,5 hours a day. (Pretty conservative estimate)
So how much would you save if you got a HD 4850 instead of a HD 4870?
How much would that SLI cost you extra in electricity bill?
GeForce 8800 GT (512 MB) €138
Radeon HD 4870 (512 MB) €149
Radeon HD 4850 (512 MB) €86
GeForce 8800 GT (512 MB) $71
Radeon HD 4870 (512 MB) $76
Radeon HD 4850 (512 MB) $44
Added energy costs of that extra card can be higher then the purchase costs at least here in the Netherlands. Also in the US this added cost is relevant.
Difference between idle 55w cards and idle 95w cards are not that big in america, and could be somewhat in line with the added preformance. However, when factoring in this added energy costs between two of such cards in the Netherlands, it appers you start paying alot extra for that increased preformance. (abeit over a longer period in time and only if you are responsible for the energy bill)
If you want to relate to this figures you should take into account the amount of time you will use the system (ie 24/7 download station or no) and what the electricity price is in your area.
On a personal note i now see that instead of getting the 80w idle 9600 GT i should have purchased the 55w idle HD 4850. The benefit in less energy consumption is €40, enough to cover the price difference between the two. I would have had a significantly faster GPU card for the family machine as well as a cooler machine and saved the enviroment.