That's a tough one. For one thing, any answer you give can place you out of the running as a viable Candidate if the company is trying to stick with a certain amount. Typically, you may want to give a Range of what you deem as acceptable. For example, whenever I'm asked this question, I respond with "For this specific Position, I find the Range to be in the neighborhood of $XXX to $XXX. What is the company's stance as far as compensation? Are they trying to keep up with their competition?" This lets the Interviewer know if you've done any investigating on your own or if you're willing to take just what they give you.

Unless they bring it up, don't bring it up. If they ask you much you made at your previous jobs, tell them. That gives them an idea of what to offer. Usually the bargaining starts when they make an offer