You dont really want to change the geometry of the whole bike by changing the wheels like that. I ride a very similar bike 06 FCR3 from giant. Almost the exact same specs. The beauty about the bike is that you really have decent flexibility in the tire sizes. I can ride anywhere from 35c-23c on the stock wheels. I have a variety of tires including road tires and some all weather, but have yet to buy any cyclocross or light MTB tires as I have a MTB for that.
I wouldn't change the weel size, just put tires that make you feel more comfortable on whatever terrain you'll be riding. Just my opinion anyways. Though I am curios as to what the limitations are to these bikes.