Well, first of all, don't drop Miller. His start last year was similar to this and he then he went on fire in the second half, winning 12 straight or something. He should turn it around and you don't want to dump him, have someone else grab him and watch them ride him to the title.
If you have to choose between Hampton and Myers, I would probably take Hampton, since he is the more proven starter, with a long history of success, and he now is playing for the Braves, who are widely considered to have the best pitching coaches and handle their staff very well and get the most out of them. He should win 12-15 games at least and is backed up by a great closer in Smoltz. Myers is unproven, has lots of upside, but could be streaky and inconsistent, and is backed up by a subpar closer in Mesa. That said, I think Myers is still a good pickup, but you don't want to drop Miller for him. I wouldn't worry about dropping Batista, he is marginal at best and plays in a hitters park for a team that looks like it will be very disappointing this year.