I agree with the previous poster: if your career goals are entrepreneurship, then there's just no reason to pursue a PhD in Business. In fact, stating those as a career goal would be the death of your application, even if you did net a 750 on the GMAT.
PhDs are for aspiring academics.
The other poster's right, MBA is a better goal, but a couple years down the road would be a better time to apply, you're still awful young. But if you get a couple years more experience and do get the 700+, you could maybe take a shot at a top-10 B-school, do Fin or OB or Entrep, and be on your road to setting up that business. If that bores you after you've done it for a number of years and you then decide you want to be an academic, in your late 20s or in your 30s (but not later), sell the company, throw the massive amounts of money you're earned from being such a good entrepreneur in mutual funds, and apply for admission to a good PhD program. That's the way to do it.
Now, if a secondary goal of yours is to teach at a research university and the entrep thing is something you'd be willing to back burner, then do the PhD, but make sure that's what you want to do. It will suck up the rest of your 20s and you will not have all that much time for anything else. Getting a PhD is not a thing you just grind away at and get like a UG degree--it's a lot of work and sweat and you gotta contribute something unique to the field and people often burn out and walk away a few years in with little to show and a lot of debt. It's a 5 - 7 year committment with nothing but a tuition waiver and a $15 - $25K stipend.
Look before you leap.