You should pursue your passion. If you're passionate about something and you work hard, then I think you'll be successful. If you start a business because you think you're going to make a lot of money at it, then you probably won't be successful, because that's the wrong reason to start a business. You have to really believe in what you're doing, be passionate enough about it so that you will put in the hours and hard work that it takes to actually succeed there, and then you'll be successful.
Source: Academy of Achievement: Pierre Omidyar Interview: http://www.achievement.org/autodoc/page/omi0int-1
Contributed by: ~C4Chaos