hosetly why wait. The atlonII X2 240 is a 65w part so the net savings over the 240e would be about a nickel a day and thats if you run at full load all the time. Idle power draw between the two is closer to no difference. At full load you'd need to run almost 2 years to make up for the difference in prices.
an i5-661 is a real power miser too, often seeing total system idle power in the 30's (watts) wit the right build and often beats the 240e at full load.
I found the article I was recalling. http://www.tomshardware.com/reviews/intel-clarkdale-core-i5-661,2516-14.html
The cost is a quite a bit more for the i5 build which you'd likely never recoup entirely.