Standard road gearing with a 25 cog in back actually works fine for cross racing. You'll spend the majority of your time in the little ring. You might wish for a gear somewhat lower than 39/25 once in a while, which is why a lot of people (like me) use a 12-27 cluster, tho I actually use a single 42 ring for racing.

One strategy that seems to make sense is to have two rings, but very closely geared, like say a 36 and a 44, and basically use one or the other depending on the course. Fast, "grass-crit" style course, stay in the 44, a really muddy sandy mess of a course, stay in the 36.