Whoops! Missed that part about the ATX PSU the first time through.
Dave is right, your ground will end up being at -5 V relative to everything else. Not good!
Better to use a wall-wart to supply the 7-9 volts you need. (Or just use the +12 volts from that ATX power supply...) The Maxyz module has a really good voltage regulator, so you'll be fine - just make sure it doesn't get too hot.
Adam