In general, the 2005 has the same engine as the 2004 but a different body. This is typical of Porsche with saving a new engine for mid-life kickers to boost sales and keep the body style selling. I wouldn't be shying away from a 2005. Go read a review from 2005 to see all the changes they made...some worthwhile. Of course style is an individual thing.
mikesporscheboxsterwebpages - mikefocke2 has a dozen articles on buying a Boxster.