which steppers to buy?

Hello again all, I am now ready to purchase the stepper motors for my
bridgeport cnc conversion. Looking at the mendonet site, I have
selected the nema 34's 720 oz. at $247.00 each. Now for my question;
the motors are listed as either 2,4 or 6 amps. The 2 amp motors are
listed at 5.4 volts, the 4 amp at 2.8 and the 6 amp at 2.1 I am
running a 72 volt power supply from Bill Griffin, with Gecko drives,
and am using the AH-HA CNC Pro software. (also from Bill) Since I
have such a high voltage power supply, can I use the 2 amp motors
(higher voltage) and put less stress on the Geckos? What is a good
guideline for motor voltage versus power supply voltage? 10 times? 20
times? Thanks for all the help; John