The terrain is the limiting factor. As far as the ram goes they are not all the difficult to sneak up on if you have good terrain for a stalk. That being said it is usually tough to find a sheep in any terrain that is ideally suited for a close stalk. I wouldnt hesitate to shoot a sheep out to 45-50 yards if I have a good solid shot!
It's not unreasonable to get within 20 yards depending on the terrain. Both my bighorn and my dall were in areas where I was able to put good stalks on several sheep. My bighorn was at 28 yards and my dall ended up being 70 yards...but I met him at four feet. It was pretty intense. However, during that dall sheep hunt, I was able to get to about 20 yards on two other sheep. Sheep hunting is just plain exciting!