Is that switch a momentary one? Or does it stay in the on/off position.
The problem I can see is if it stays in the 'on' position once you've turned the switch then your pc will boot, and a few seconds later it will turn off (because the ATX switches only require a momentary connection to switch the pc on, and the same to turn off).
Ideally look for a switch that is momentary. There are key-based ones on the net that are spring loaded, so you put the key in, turn it round to 'on' and then it springs back to 'off'.