I don't know what max pulse rate is for a Galil card and Mach3. But it doesn't make much difference if you run steppers at 1/4 micro step or 1/30 micro step as long as you can supply enough steps in one second you will have a some high rapids.
On one of mine Bridgeport mills that I retrofitted with Mach3 it has a 200 step/rev motor with a 10 microstep drive with a 2:1 reduction and a 5 turn screw this gives 20,000 steps per inch and it does ~60 inch/min max travel speed this setup will give you .00005 resolution with plenty of speed on a 4x4 machine.
There is now way to change micro step resolution because the CNC software has no way to know when the drive changed from 1/4 to 1/16 or so. And there is no need for something like that.
Take a look at some of the videos here:
http://www.romaxxcnc.com/video.htm#I'm pretty sure they are running mach3 and I would say that is plenty of speed for 99.8% of the projects and people.
So do you want your machine to be very fast or very accurate? With the Galil card you have it will be both and then some.