1) Very
2) If you hit 250, you'll be in great shape.
3) I think the AMA has a fact page with this info.
4) Lots. But if you're in it for the money, you may not be able to make it past med school admissions interviews. Let alone, the 4 years of med school. Then the 5 years of residency. Then the realization that with dropping compensation, you might have wasted 9 years of your life in pursuing something purely for money. (Not preaching...just hoping you have some other reason for doing this than $$$)