Gotta Love "Bright Ideas"

I had one of these "Bright Ideas" the other day, and now it seems it's going to evolve into a GIANT project.  I made the mistake of talking out loud to my wife about the fact that our house was going to be a little tight if and when we have a second child.  My son is turning 1 at the end of the month and our house is too small to hold the party, so we're using my parents place instead.  Don't get me wrong, my house is nice, it's an older Cape Cod, 3 bedrooms, 2 baths, full basement, blah blah blah. 
My mistake came in wondering out loud about putting an addition onto our house.  Now I'm looking into bringing in contractors and having them quote the framing, roofing and foundation work.  I can handle pretty much anything else (wiring, plumbing, flooring, drywall, etc).  We're looking to add like 600 square feet to the house, so I've got a lot of work ahead of me. 
Anyone gone through this in the past?  Please give me some advice!  I'm not even sure where to start.  Home Equity loan first, then see what it costs, or get the quotes first, then go to the bank?  Can I find out ahead of time what the addition will do to my property taxes?  Permits?  AUUUGGGGHHH.  Screw it, I'm going to play Dead Rising and kill me some zombies.

Sheesh