i'm not familiar with the boards that you are using, but i can tell you about pc powersupplies.. first, no you cant use the + and - to get your 24v.. if you look closely at the specs on most AT and ATX PSU's you will find that the - supply is very low current, on the ones that i have play'd with it is in the .5A range..
if you do some research you can find instructions to convert pc psu's for use as bench supplies, if you break the connection of the ground to the case, and to the ground side of the outputs, you can then daisy chain a couple of them to get your power, but be very carefull if you go this route, there are some large caps and high voltages in these things, that can really hurt or worse...
my suggestion would be to go to a more conventional type powersupply. you can get these on ebay for around $30 and up depending on the specs that you need..
as for current. i dont know about your boards again, but just for compairson.. the gecko's reccomend around 2/3 of the motor current per motor, so just for example, if your motors are 2.1A then that would be 1.4A per motor, then multiply by the # of motors that you have.. assuming 3 that would be 4.2A, plus a little safety margin, i would call it 5A... or thats my understand anyway.. i might be way off, but thats the math that i'm using to set mine up..
Grizzly X3, CNC Fusion Ballscrew kit, 3 500oz-in bipolar steppers, 3 203v Gecko's, Linear power supply from Hubbard CNC, Mach 3, BOBcad Pro Art V22, Rhino.