nothing displays on the monitor when I reboot (although Windows 7 comes up)
If you connect the cable to the card can you see the white text during boot and can you access and see the bios settings?
Based on what you say it seems you do. If that's the case there must be some driver issue preventing Win 7 from working with the new card. Remove the 550. Leave the cable plugged on the mobo (so that you are using the IGD) and uninstall all that came with the old card, if it was NVIDIA, have a run with driver sweeper
(be sure to remove only display drivers).
Then turn off, place the 550, leave the cable where it is. Install the drivers for the 550. Turn off again, and place the cable on the card (and the bios option to use that card), and you should be ready to go.