|"I am thinking of disabling the onboard adapter in device manager"|
Do NOT do that! You should have uninstalled the onboard video drivers before installing the card. After the card was installed & the monitor cable connected to it, you should have booted up & immediately gone into the BIOS to change whatever setting is necessary to disable the onboard, return the shared memory to the system, change the initial display to the PCIe slot, etc. We can't see your BIOS screen but there should be some sort of option to disable the onboard.
Did you click on the HP link I provided? Step 1 = remove video drivers. And later in the explanation = If your PC was using on-board video (no video card), then disable on-board video in the BIOS to prevent possible setup problems
You also haven't provided any info about your PSU or the exact model of your system (p6000 is the series). It's unlikely the PSU is the problem but you never know. You definitely didn't need a 550W PSU for the lowly GT620/GT530 but did you upgrade them both at the same time or have you been using that PSU for a while?