Get something real up and running quickly. Running software is the best way to build momentum, rally your team, and flush out ideas that don't work. It should be your number one priority from day one. It's ok to do less, skip details, and take shortcuts in your process if it'll lead to running software faster. Once you're there, you'll be rewarded with a significantly more accurate perspective on how to proceed. Stories, wireframes, even html mockups, are just approximations. Running software is real. With real, running software everyone gets closer to true understanding and agreement. You avoid heated arguments over sketches and paragraphs that wind up turning out not to matter anyway. You realize that parts you thought were trivial are actually quite crucial. Real things lead to real reactions. And that's how you get to the truth.
And on that note, this blog is going away soon. I'm going to be needing this space for something else.