It's always difficult trying to judge savings on power efficiency. There are a lot of variables (local electricity costs, typical load, time). I would be a little surprised if one would save $200 from a year's saved electricity from the Penryn chip. $100 maybe, and perhaps that's enough to justify getting the E8400 anyhow.
If you wait until Q3, you may be able to get a decent quadcore if the rumoured Intel price cuts come true. The Q9550 is supposedly going to drop to near $300.