Start on a 600cc? Nah-ah, no way. Start on a 600cc and you'll be the next twitching body video on YouTube.

Don't know what they do in America, but on this side of the pond, people who are starting out usually start by riding a scooter. From that they usually go to a 125cc bike (which you might be able to start on, actually), then up to 500cc, and then you can do what you want.

Even though you've driven dirt bikes, I'd still try a 125cc before you go crazy. It might be different in America again, but the places over here that teach you to ride bikes usually have their own 125cc bikes that they teach you on in private yards, before moving onto the roads with a 500cc bike.

There's no point jumping in at the deep-end with bikes - unlike cars, you're not protected if the prick in the Beamer decides to pull out in front of you, and you can't control the bike because it's too powerful for you as you've never learnt how to handle it. Take it slowly. Sure, it might cost a bit more because of lessons, but you'll be glad of it.