There are plenty of sites on the internet giving dosages and types of chocolate to sicken and kill. Here's the first one from a google search: [link|http://www.dogownersdigest.com/news/library/chocolate-dog-poisoning.shtml| Chocolate Dog Poisoning].
On a recent trip to the US, I brought a box of chocolates as a gift. The family mutt (13 yo. chocolate lab...) burgled my suitcase and absconded with the whole pound. No ill results other than a brief spell of the runs and mutt absolutely want to know where the rest of the goods are...
For humans, it acts as a mood enhancer. Eating chocolate really does make you feel good. Comined with the caffeine found in cocoa, chocolate acts as a pick me up. Watch your coworkers head to the vending machines for a candy bar as they hit their mid afternoon slump.
Dogs and horses metabolize theobromine slower than we do- which is why its more toxic to them. It over-stimulates the heart and lowers blood pressure.