Not only does it effect their hearts, but they can go into "Pancritius Shock" (SP).
If anyone remembers my ex's family dog Cadeau (BM, mix), she got into chocolate before I knew them.. My ex said she was shaking, foaming at the mouth, & puking all over... he said it looked like she was having a seizure!
Luckly she made it to the vet on time & lived. But that was pure luck that she survived it after getting that bad.
Just like people, some dogs can handle chocolate longer/larger amounts the other dogs, but that aside, its still poison for them.