ok first of all, if the muscle is a still a little sore after 4-5 days of resting, its probably something you did wrong last chest day.. also as far as " OVER TRAINING " is concerned, myself and many others can work each body part twice a week and not over train. It all depends on your body, no one is the same, so overtraining for you, wouldnt be for me or necessarily him or anyone else.