Hi Paul!
You really shouldn't pick your motors until you know what drivers and PSU you will be using. Everything has to work together. You will want to wire the motors Bipolar Parallel for best results.
Voltage = power and power = speed. Formula for best motor voltage is 32 times the square root of motor inductance. Try to get as close to that as possible.
Many drivers restrict max voltage. In that case, you should try to find a motor with best voltage equal to or slightly above that restriction.
ON my SX3, I used Keling 425s for X and Y. Their best voltage is 85V, so I used G203s and a 72V PSU. These give good performance--250 IPM rapids, But there are better motors available now.
Today, I would use $49 65V Keling KL23H284-35-4B 387s (back in stock by 2/10/09) for the X and Y, and a $89 640 OZ KL34280-45-8A for the Z axis. I would power these with the $129 KL5010 power supply. I would again go with the G203Vs because there is no better driver anywhere.
If there were a 640 oz N34 motor that would fit it's parameters, I would definitely go with the much less expensive G540, which is a Jr set of 203Vs.
http://geckodrive.com/product.aspx?c=3&i=14460 http://kelinginc.net/KL23H284-35-4B.pdf http://kelinginc.net/KL34H280-45-8A.pdf http://kelinginc.net/SwitchingPowerSupply.html
When you are buying the N34 motor, remember that only a single stack motor will fit in the space under the column.
CR.