That is about the size steppers I recommend for an X3. Remember that your stepper drivers, power supply and steppers and lead-screws need to work well together for best performance.
With only 10 tpi lead screws you will want a belt reduction to get the steppers operating with enough torque at the speeds you will be running. Somewhere between 1.5:1 to 2:1 would be good.
A good combination is 276 oz-in steppers, a good 48V power supply, and a G540:
http://cnczone.com/CNCParts.aspx .
