The fit of the G540 is dependant upon the type of motors you have, how they are wired, and what performance you wish to get out of them. Take a look at the G540 docs:
http://geckodrive.com/product.aspx?c=3&i=14469 to get an idea of the ideal range of motor sizes / inductance for the G540. Gecko also has a good guide to steppers at:
http://geckodrive.com/upload/Step_motor_basics.pdf, starting at page 10 it talks about power supplies. For the G540 sized machines I prefer to use a switch type power supply as they are more compact and much more efficient (for instance the model I sell is more than 90% efficient), they also generally have over current, over voltage, over temp protection built in. They also have a formula floating around somewhere for doing a rough calculation on the 'ideal' power supply voltage.
Depending on your machines configuration there may not be a single 'ideal' power supply voltage. I just built up a special STDR-4C drive system (G540 based) for a customer that had a 4'x8' commercially made router. The stepper motors installed at the factory were all over the ballpark as far as size/inductance. Gecko had recommended they replace their existing NEE controller with a G540 and after finding out the motor specs and how the motors were wired it was apparent that there was no one 'ideal' power supply voltage (or driver for that matter) but the G540 was a good fit and was no doubt much better than the original drivers. I'll have pictures of the custom STDR-4C up on my website soon.
One thing to keep in mind is that the G540 has a fixed 10x microstepping. From a performance standpoint that works out great as it gives very smooth low speeds and as the G540 morphs to full steps as the RPMs increase higher RPM performance is good as well. The step morphing is an internal function, you just set Mach's steps / unit for 10x microstepping. If you have a lot of gear reduction and a slowish PC it could have the potential to cause problems if your LPT can't generate pulses fast enough. Usually it is not the case and increasing the pulse time or selecting Sherline 1/2 pulse mode will get it working fine. On my Taig mill the steps/unit with a G540 comes out to 40,000. My old shop PC can run the machine close to 90 IPM with the SS it can go faster.
If you can find out the specs on your motors someone may be able to give you a more exact answer.