Big Dreams - a waste of time?

If you're a semi-regular reader of my blog you'll know that I'm a great believer of having goals and targets to meet when you setup in business. As I mentioned in my mini series on starting up in business, these targets have to be achievable and quantifiable. Recently however I have had a couple of pretty zany ideas passed by me which, at first sight may sound completely nuts and unachievable but I've been left wondering why that is. I have always felt that The Site Doctor is a foundation for something larger but quite what that is I have no idea at the moment but one idea that's readily stuck with me is one that Rich Davies and I merely jested about -buying a big, expensive yacht and sailing it around the Caribbean for the rich and famous. I've put this idea to a couple of people since and every time they've laughed. I can understand why they see it as a joke but why should it be? When I first setup in business I had a surprising number of people assure me I was mad and it would never work so why should this idea be any different? Ok the investment involved is on a scale way out of the league of The Site Doctor but I still don't feel it's unachievable if the time and effort is put into it. What do you think? Is thinking big just a waste of time in your eyes or is it more speculative than wasteful?