Hi,
 
I'm about to start my first system build, and I'm hoping to keep it as energy efficient as possible (more for moral reasons than financial ones).
 
E7200
Gigabyte EP35-DS3L
MSI 9600GT
8 gigs OCZ DDR2 800
640 gig WD Caviar HD
22" Acer LCD
Asus 22x DVD burner
 
this is all going into an Antec Sonata Designer case that came w/an Earthwatts 500W power supply.   My question is how much overkill is that for the system?  I bought the case because it was a great deal ($89 after rebate), but will it be running really inefficiently on this system?  It's 80 plus certified, but I'm guessing that a 300W supply would do just fine.  Is it worth swapping out the large supply for a smaller one?

8 gigs is a bit much unless you are doing some rendering and what not. You could just go for 4 gigs and that will save a little power to make the world a greener place.

a 500w power supply does not use any more power than a 300w power supply does when powering the same system.  
http://www.hardwaresecrets.com/article/181
Keep the high efficiency 500w earthwatts.

Thanks, dm.  I hadn't seen that article before -- I'll be reading it in depth now!

The PSU might be more than you need but it has very good efficiency down in the ranges where your system will run.
PC Load / Eff % / Draw from wall socket (From: HardwareSecrets review )
193W /  87.7% /  220 W
295W /  86.5% /  341 W
You'll need a PSU that would beat those figures and that might be hard to do.

edit;
For comparison:  Zalman ZM360 PSU  Corsair VX450  * The Corsair is fractionally more efficient than the Antec EA500.