Have you tried installing the new drivers before you take the old card out then disabling the drivers of the old card?
Having done that have you tried powering down the PC and disconnecting the monitor before opening up the PC case? When you have fitted in the card have you checked that the HDMI lead is connected to the new card (if you have more than one option)?
I assume thave you have a CD with the new card. After connecting the monitor and booting the PC does anything appear on the screen after you insert the driver CD for the new card?