Hi Toby! If those motors are Homecnc 23-205-ds8s then you will need a 48 - 50V PSU that outputs at least 10 Amps. The $129 Keling KL-5010 would be just right.
http://kelinginc.net/
One note about these motors. They are 205 oz in Unipolar. But with a G540 you will want to wire them in Bipolar Parallel. In THIS configuration they are 4.2A 285 oz motors. The G540 can only accept 3.5A max, so the actual rated torque will be about 240 oz.
The motors you have are not the most efficient choice. Although, if you are running with 2 motors slaved to the X axis, They might work well there. It is customary to use a slightly more powerful motor on the Z axis because it has to lift weight against gravity.
If I were advising you from scratch, I would recommend this:
$299 G540
http://geckodrive.com/product.aspx?c=3&i=14469
$117 Three $39 KL23H276-28-4B 270 oz motors for X & Y
$049 One KL23H284-35-4B 387 oz motor for Z
$129 One KL-5010 power supply.
$010 Estop switch
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$604 Total electronics cost.
$149 Mach3 ($25 discount) cost with above purchase.
Keling has all of this, and ships at warp speed.
http://kelinginc.net/
CR.