You should get the /minimum/ voltage required at the amperage your motors can take which will combine (amps * volts) to produce the wattage (power) required to move the load you need to move. This page techref.massmind.org/techref/io/steppers.htm#Estimating explains that fully and helps you calculate what you actually need.
The Linistepper design in general, and the kit for sure, is rated for a maximum of 32 volts... 36 if you push it a little. Because it is a linear driver, you need at least a big heatsink and more likely a CPU cooling fan if you run at 2 amps to keep the drive transistors from frying.
This idea that more voltage is better is just pain wrong. Applying too much drive power to a motor can make it slam from one step to the next, causing vibration, resonance, and mid band problems where the motor can miss steps. Measure the load, and use the /right/ power to drive the system, not just more, more, more!
James hosts the single best wiki page about steppers for CNC hobbyists on the net:
http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)