What about the +4, the extra 4 pin plug aside from the main header? I have never tried to use a system without these installed.
you won't hurt anything attempting to power the board off only 20 pins. if it is ok with that, it will boot and run as normal. if it actually requires all 24 pins, then it shouldn't even boot. the reports i've heard say the pc won't POST, but i could never find hardware to verify those reports. i used to work on a lot of old pc-based camera dvr's, and while i know some boards exist that require all 24 pins, in my 3 years repairing hundreds of dvr's, i never found a board that didn't work off of only 20 pins.