Well, I had one of those dreams when I was 23. When I suddenly woke up, I was thinking: what if we could download the whole web, and just keep the links and... I grabbed a pen and started writing! Sometimes it is important to wake up and stop dreaming. I spent the middle of that night scribbling out the details and convincing myself it would work. Soon after, I told my advisor, Terry Winograd, it would take a couple of weeks to download the web -- he nodded knowingly, fully aware it would take much longer but wise enough to not tell me. The optimism of youth is often underrated! Amazingly, I had no thought of building a search engine. The idea wasn't even on the radar. But, much later we happened upon a better way of ranking webpages to make a really great search engine, and Google was born. When a really great dream shows up, grab it!
I think it is often easier to make progress on mega-ambitious dreams. I know that sounds completely nuts. But, since no one else is crazy enough to do it, you have little competition.