Yeah, I'm currently going through the ELH monsters trying to make them a bit more, well, epic, which is how I came to ask these questions.
Gah, really? What are those?
With DR it depends how much your players are optimizers. Against a reasonably high-OP group 3.5 DR might as well not be there, but against a low-OP group 3.0 DR means that some characters can't hurt them at all. Our group is what I'd probably call the lower end of mid-OP, so the 3.5 DR is about right, with a few exceptions (the Hecatoncheires being one of them; DR20 against a 57th-level party, really?).