I'm having a similar problem. With the ADD2-N card, if I have a monitor plugged in at boot time, the system won't start. However, if I leave my 20" Cinema Display unplugged, startup using a VGA based monitor, then plug the Cinema Display back in when OS X finally begins loading, I can switch monitors.
Once I figure out how to fix the startup problems, I'll be sure to have an answer. For now, just try doing what I suggested.
For reference, my board is an Intel D915GAVL. It uses the i915G chipset, as far as I'm aware. The ADD2-N card is an ASUS supplied one, but is also a Silicon Image one.