Very little I would say.
The capacitors in these things pack a good punch (not enough to kill you) but will be enough for slight burns and a bit of pain
Wear some rubber gloves if you can, you'll be fine.
http://www.instructables.com/id/Convert-an-ATX-Power-Supply-Into-a-Regular-DC-Powe/
http://www.rcuniverse.com/forum/bat...0951-how-convert-atx-power-supply-rc-use.html
Edit: PinkFloyd is right however, if you wanted a proper PSU (with a switch, power light, banana plug jacks etc) then you need to open it up.
Disclaimer: I shall not be held responsible for any death, injury or suffering you, your family, pets and sheep may be subjected too following those two links