I just bought a completely new setup including boots, since all my gear was around 8 years old. I made some mistakes, so hopefully you can learn from them. Then main problem I had was that at first I was only going to replace my bindings, so I bought those first. Bought a moderately flexy binding to fit my old boots. Then I bought a new board, because I figured my board was old technology as well. Then, I went snowboarding on my old setup last weekend and broke one of my boots, so I ended up with new boots.
The lessons I learned:
Buy boots first. IMO they are the most important piece. Try on at least 10 pairs and at least 5 different brands. Know whether or not you want a stiff or soft boot.
Buy bindings and the board after you buy your boots. If you bought fairly stiff boots, you probably want pretty stiff bindings and a moderately stiff to really stiff board. Same goes for soft boots/bindings/board. If you're a beginner, look for a soft to right in the middle setup. Do your research. Look on the websites of different companies to find out which boots and bindings are soft or stiff.
As far as money goes:
Boots: Don't skimp here, and I wouldn't order online unless you've tried them in a store. Be aware that they might change from year to year, so trying on an '09 and buying a cheap '08 online might not be the best thing to do. I spent about $280 after taxes, but only because those were the best fitting boots. If I had found $150 dollar boots that were the stiffness I wanted and fit right I would have bought them.
Bindings: These you might be able to pick up for a good deal online after doing your research. Not sure what is still available at this point in the season, I originally bought some online for cheap, but I'll be returning/selling them because they aren't the right binding for me. Had to drop $200 after taxes on '09s.
Board: Same as bindings. A few weeks ago, there were a lot of '08s on clearance. Not sure what is available now. I spent about $240 here on a $430 retail-price board.
Total of everything was about $720, but could have been less if I had been smarter about the bindings.