Let's see. Using Han's assumptions, if you game for 100 hours/month and if your computer is otherwise on for 200 other hours, that comes out to:
100 hours x 100 W = 10 kWh
200 hours x 25 W = 5 kWh
For a total savings of 15 kWh per month. At ten cents a kWh you'd save $1.50/month off your electricity bill, or $18/year. If you plan to use the card for two years, it would be a difference of $36 over two years. Thus, it might well be a better deal than whatever the 8800 GTS 320's end up on sale for (ignoring performance issues).
That's not earth shattering, but still, it's nice to know that the price of the new cards will be a little less than what they appear on paper relative to the price of previous cards.