how much do you think used 2004/2003 M3's will drop, now that the new 2008 model is out?

Typically in these situations, prices on a vehicle that old will not drop, you will just have a better selection to choose from. The new '08 models will affect the '07 model prices for those cars still on dealership lots, but it's likely folks who kept their M3's off-lease or from outright purchase for 4-5 years will not be overly pushed to sell them to get a newer model.