It sounds like your commute is under 15 miles. You may well enjoy the quickness of a road or cyclocross bike, but when all is said and done, you might not get to your destination all that much faster. Just keep that in mind. I've used all kinds of bikes on my 6 mile commute. My road bike is definitely the fastest and the most fun. Still, at my normal level of effort, it only saves a few minutes.

Any shops in the area that would let you rent a road bike? You could see if it makes a real difference for you or not before you spend a lot of money.