This is something I'm also struggling to get my head around. For example...
I only have a 3mp camera, so if I want to print at say A4 size and I change DPI to 300. Will this alone give me a better print or should I resample the image too to give me more pixels?
Sorry if I appear a bit thick on this. 

www.natural-photo.co.uk

"Dodging and burning are steps to take care of mistakes God made in establishing tonal relationships.  ~Ansel Adams"