I'm not finding much on this model of HP other than the HP site, In the trouble shooting section it has a issue that your monitor is blank and gives some things to try, One is either update or reset the bios in case you accidentally changed something LOL Well it might be worth a try, This is how they say to do it on your PC without the monitor working. I'd suggest you put the 660 in and try this so it detects it's there and opens the port for the 660 and not the on board. To see which version bios you have it should say someplace in the bios but sometimes can be hard to find but it's in there, Also read all you can when going into the bios, It may flash which version when it's starting.
Reset bios version 7 or greater
1. Turn on the computer and repeatedly press the F10 key for eight seconds.
2. Press the Down arrow three times.
3. Press the Enter key two times.
4. Restart the computer.
Reset bios version 6 or less
1. Turn on the computer and repeatedly press one of the following keyboard keys. Press the key about once every second for 5 seconds after turning the computer on.
For HP and Compaq computers that came with Windows 7 or Windows Vista, press F10 .
For HP and Compaq computers that came with Windows XP, press F1 .
For Compaq Presario desktop computers built prior to 2002, press F10 .
2. Wait another 5 seconds.
3. Press the F5 key. This restores the default settings of the BIOS.
4. Press the Enter key. This accepts the changes.
5. Press the F10 key. This saves the changes.
6. Press the Enter key. This saves the changes and exits from the BIOS.
7. The monitor should display the first screens received from the computer (the logo screens). If not, find and download the latest BIOS update if one is available for your computer.
8. If a BIOS update was not available or the monitor is still blank, continue to the next step.
This is the website that has a bunch of things to try also if the BIOS doesn't get it going.