Scott, I'm curious where you are getting those numbers from. Are you taking the "4 drives at 3 amps each" from the G540 specs? If so, keep in mind that cd may not be driving his motors at the G540's full capacity. E.g. he might have a 1 amp/phase motor and so set the G540 to a lower than maximum rating. Having said that, it's a safe bet if you give the board all the power it can ever need, in case you upgrade motors in the future.
Also, manufacturers can rate their motors under different systems. Most of the old round unipolar motors you pull out of old equipment have label ratings for 2 phases on. So 1.5A means both phases always running at 1.5A. But newer motors (like most square ones) are rated per phase, so 1.5A means 1.5 amps maximum with one coil running.
So, as I understand it, you need 3 amps * 0.67 (if using a chopper type driver) to run that motor.
More information at:
techref.massmind.org/techref/io/stepper/power.htm
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)