Robot Arm POWER SUPLY is ok, help with MOSFET'S???

hey guys i need some help from u, i had just ended manufacturing all the parts for may robot
my robot has 6 steppers it simulate human arm movement
5 of the steppers are 103-546-8441 http://www.ebay.com/itm/Sanyo-Denki-103-546-8441-Stepper-Motor-Tested-/181147365316 24v 1A
i made a planetary gear that works very well with a reduction of 81 or 243 to 1, to use smaller steppers and high speed
the control for steppers is ok i had used 4 atmega 644 ...

problems
low budget!!!!
the drivers are made by me :)) with 2 uln 2003 with 3 outputs in parallel see pictures, 3x500ma=1.5A
how do i calculate the power consumption in W, needed for steppers ...
are relayeble the pc PSU, i wish to modify 2 pc PSU to output 24v
other project problems that i am not aware of

The motors you have pictured are bipolar (four wires) and you have a unipolar (five wire) driver.

yes yes i know that, SORRRYY my bad ... i have lots of scrap steppers ... different types and manufacturers, i posted the first one i put my hand on.
yes i have 4 drivers for 4 unipolar steppers ...

i had found two ways until now,

1,
use one atx psu the +12v and -12v .. the difference is 24v ...

2,
two psu in series..... output 24v

ok now i have the 24V power supply i was really nervous this will be a problem but it was not
i used those sites

http://www.cnczone.com/forums/general-cnc-machine-related-electronics/17548-daisychaining-2-atx-computer-psu.html
http://www.procooling.com/index.php?func=articles&disp=52&pg=2

http://thedatastream.4hv.org/at_psu_hack.htm
now i need to make a nice casing

now

who had played with FET's and steppers ????
i wish to replace the uln2003 with some power mosfet
from what i know, i need logic level fet's 3.3-5v, fast switching ... em i right?
why every fet must be isolated from the heatsink if i use comon one... ?? if all are the same , the heatsink is to ground ??
what else i must consider ??? for a 24v unipolar stepper
i have sow much mosfets's from where to chose, its really hard when u have a grate verity,

IRLU2703PBF http://www.tme.eu/en/Document/ab2f12c7182ed5843b7e308302cb74ff/irlr2703pbf.pdf
IRLU3103PBF http://www.tme.eu/en/Document/162937161ace2cd9941961726d763b7b/irlr3103pbf.pdf
IRLZ44 http://www.tme.eu/en/Document/71cf1899624764088671ce6de5d15eb4/irlz44n.pdf
nte2980
IRFP4410ZPBF http://www.tme.eu/en/Document/dcff7bec0e14e08117b6eb2a6c429f2c/irfp4410zpbf.pdf
irf540