First point is that the voltage rating on a stepper motor is usually of little interest as its rarely meaningful - for your motor the voltage is not 24v but the product of the current and the resistance = 6.5*.21 = 1.37v parallel or 2.8v series which suggests a supply between 6 and 67v. The voltage stated is likely to be some recommended upper limit of drive voltage to avoid insulation breakdown.
Current = torque, so what is of primary interest is the current you can drive it with. Where the power supply voltage comes into the equation is where the top end of the torque is in terms of speed. A stepper motor has roughly fixed torque up to its corner speed, which is determined by the coil inductance and the supply voltage. Above the corner speed it is a constant power motor, power = torque x revs, so as the revs go up the torque drops off. Ideally you want the operating speed of the motor to be at or just below the corner speed.
A rough rule of thumb suggests the driver voltage should be 32 * sqrt(L) where L is motor inductance which for your motor = 32*sqrt(.7) = 27v in parallel (53v in series). So 24v is perfectly sound for that motor when operated in parallel mode. In fact the corner speed (in parallel) is around 1000rpm (490rpm series) so as long as you're running the motor below that speed you'll be getting the best out of it and there would be little value in going to a higher voltage.
As you can see, parallel mode gives the best torque v speed and is the recommended mode of use.
If you're in Europe why not come and visit the UK CNC Community at http://www.mycncuk.com