They are planning to put a twin turbo 6 in the M3 but it won't be until at least 2012-2013, don't know about HP rating but 450 doesn't seem far fetched. I prefer a V8 over anything with a turbo. I suggest you buy now going to be a good 2 years before anything major changes.
2010 E92 M3 - Alpine White Mine on 11/2/09 CarDomain
Coupe - Sold