Minimum hardware requirements to get live display?

Just wonder which are the necessary hardware components to get any kind of image/text on the display. I have figured these out myself, but what more do I need?
Mobo, RAM, CPU, PSU/case, display.

Do I need a video adapter when I just want to test if the display and other hardware works?
Do I need a hard drive?
Anything else?

And sometimes a keyboard...I've heard some boards won't respond if they don't detect a keyboard, but that was a few years ago.

<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>