This is a very good question, but unfortunately (like most things in bodybuilding and fitness), it doesn't have an easy answer...and the answer is still debatable.
As someone previously said, dumbbells are going to require you to use more small, stabilizer muscles than a barbell (or machine) chest press will.
Thus, if you normally bench press 100 pounds for 10 reps, it is very unlikely that you will be able to do 50 pound dumbbells for 10 reps...40-45 is more likely, less if you've never done a dumbbell press before, possibly.
Does a dumbbell bench press provide the same amount of muscle stimulation as a barbell bench press? Here are some trains of thought:
1. Since dumbbells require more stabilizer muscles, some of the focus on the prime movers (pectorals) is lost.
2. Furthermore, since you aren't able to use as much weight on dumbbells, the principle of progressive overload suggests that this might not a thing.
1. One of the functions of your pectoral muscles is to bring your arm across your chest. This is impossible with a barbell, but you can definitely do this with dumbbells (I do this).
2. Many bodybuilders claim that they feel that the dumbbell bench press built their chest better than the barbell bench press.
3. Strength conditioning coach Bret Contreras did some EMG studies on chest exercises, and he concluded that dumbbell bench presses were a very effective chest exercise.
Article here: http://www.t-nation.com/free_online_article/sports_body_training_performance/inside_the_muscles_best_chest_and_triceps_exercises
So, even though it's debatable, I personally think that dumbbells are a better way to go if your goal is bodybuilding. However, if you are a powerlifter, you have to barbell bench, lol.