[ger21]

You probably don't want to run those in full step mode. Use at least 1/4 step mode. The reason is that microstepping reduces resonances that may cause lost steps.

You can run that board with TurboCNC, and easily get 10,000 steps/second probably over 20,000. (depends on your computer). Using Mach2, and a 1Ghz computer, you should be able to get 35,000, maybe even 45,000.

A bigger problem with steppers, is that as the motor spins faster, torque can drop significantly. By gearing the motor to spin faster, you may not see any gains, and can actually lose torque. THe best way to prevent this is to supply higher voltages to the motor. Usually, doubling the voltage will double your high speed toque. Since the SalemControl board is limited to 35V (You probably shouldn't use over 30V), that may be a factor Using Gecko drives you can use up to 70V. And up to 7a as well. This will allow you to use bigger motors.

After reading your post again, you said 48" in 10 seconds. Most machines speeds are given in inches per minute, so your speed above would be about 288ipm. The salemControl board is relatively new, and I haven't read much info on it. But it is basically the same as a Xylotex (same chip, same ratings), and typical speeds with a Xylotex and 1/2-10acme are probably 30-60ipm. This may be with a little more torque than you need, so you may be able to go a little faster. I don't know about 300ipm, though. You'll be limited to about a 250oz-in stepper with those boards, as well.

Use the link Arvid gave to calculate speeds and torque required, and then go from there.