A simple and quick check on wikipedia defines detent torque as the torque required to move the rotor between adjacent poles with no power required. In a stepper motor with permanent magnets there is a tendency for the rotor to lock onto pole pieces due to magnetic attraction (even with no power applied). The torque required to overcome this lock is the detent torque.
The example stepper you quote has a rated torque of 68ounce-inches (Oz-In). When you apply conversions to transfer from Imperial to Metric this works out as 48Newton-Centimetres (N-Cm), which is the holding torque. On the basis that holding torque is the capability of the motor then this is what the motor can exert to move the load.
As to motor speed, that is determined by the pulse rate you
apply to the motor - it is a STEPPER, in other words it steps between poles and the step rate is defined by the step frequency you apply. In the example motor it is specified as requiring 400 steps per revolution so if you apply 5000 steps per second, the motor rotates at 5000/400 = 12.5 revs/sec or 750RPM
However there is a pay-off of torque versus rotational speed. If you go to the shown link you will find a graph of how this varies, and it looks like somewhere around 50RPM is your required "sweet spot"http://www.velmex.com/pdf/mc/Vexta%20PK245%20tech%20data.pdf