Right.... I see what you're trying to achieve, but it won't work on 99% of MOBO's..... In most cases having that cap across the start switch will either shut the mobo down, or put it into standby after a few seconds.
From my site:
"A capacitor's discharged state is shorted. When you apply power to the unit,
the cap acts as a short between the pins, activating the power supply.
With 5V flowing through the cap, it's shorted state becomes an open state,
allowing the board to boot normally.
When you power down the unit, the capacitor will discharge, and resume its shorted
state, allowing it to do this the next time you power up."
I've done this several times now with no problems.