There are a lot of different types of cajons, but the traditional Flamenco style you're talking about with the sound hole in the back, I find, sounds nice with a SD condenser (KM84 or Josephson C-42 are what I usually use) in front about 12"-18" out to capture the hand and finger sounds and a LD condenser (U195 is what I use but I don't have a huge selection LD mics) in back about the same distance from the hole to catch the resonance and bass of the drum (adjust the angle of this mic toward the hole or floor to get the best balance).
Alternately, I've also had luck using the U195 and placing it off to the side about 3' away from the drum (as gtoledo3 says this lets the sound build up in the room a bit so your room should sound good to use this approach). The drummer's touch will have lot to do with exactly where you place the mic.
If you're room doesn't have a wood floor, I suggest putting the cajon on a piece of 3/4" plywood so you can get some reflection off the floor. Carpeting, IME, sucks the life out of the drum.
Jeff