It doesn't sound like you are sending the proper resolution from the card to the plasma. You need to work on that -- it's not a 'software adjustment issue.'

I have never heard of a PC needing any monitor plugged into it in order to boot up, much less a VGA requirement. However, since you say that you added the ATI card, look into disabling the PC's integrated graphics (I bet it has that) via the hardware devices list. Then try a reboot.