Its kind of late to be buying CS4, especially if you have CS3. Rumor mill says CS5 is around the corner, and that means Adobe might cut off ACR upgrade support for CS4 once that happens. ...but if you don't plan on getting a new camera body in the next couple years, and can get CS4 for a good price, then I guess the differences are something to consider. I skipped CS3 though, so I don't know.