I've lived in both areas, and used to commute between Denver and Boulder - sometimes just with the bike, sometimes with a combination of bike and bus. Broomfield was the halfway point, and the community has greatly improved its bicycle infrastructure in the past decade.

It's true that the air is pretty thin, but it's also not a hilly as you might expect until you get west of Boulder. A triple crankset would be overkill unless you're going into the mountains.

It's also quite dry in the area, so even though you'll probably be cold coming from California, the cold doesn't totally destroy you like East Coast cold. As others have mentioned, it doesn't usually last more than a week or so. I did just fine without any technical winter fabrics (dryer air meant less sweat, for me), but the choice is yours.

In fact, the weather can change wildly in the course of a single day, which means that it's important to be prepared for anything during the months of September through May. The biggest danger, I think, comes from being unprepared for the possible temperature shifts: it can be 70 degrees and sunny when you leave for work, but cold and snowing when you come home. I learned to always bring something waterproof, a midlayer, socks, and gloves.

My own experience was that a regular road bike was fine about 90% of the time. For the remaining days, something with wider tires and good traction (studded tires) works well. In many cases, the side streets will not be plowed - they just wait a week for everything to melt (unlike the Upper Midwest, where it remains a sheet of ice for the next four months.) A Cross-Check or Steamroller will work great: just be sure to get a good lock, because everybody will want your bike!