How are you trying to Start these MB's outside the case?
Just flicking the switch on the back of the Power Supply? (normally will not work) (this will work , once you boot to BIOS , set power ON after power fail).
You have to short the pins to start a MB (unless it is newer and actually has a Start button on the MB)
I removed the power button from an old case to plug into an older MB to fire it up.
Edit : all the header pins are located along the lower edge of a MB ..they are labeled , Speaker, PW SW , Reset, and the like . all that is necessary is to short the 2 pins that are marked PW SW (Power Switch) with a Screw driver or any metal object. 2 days ago i use a screw that was laying around, (to lazy to get up and get the Switch).
Edit 2: I missed this in your first post
When I fire up the motherboard, all fans run normal but simply wont post any video to the HD5780.
Thought you never had any post of any kind. But you wont get beep codes unless you add a small speaker to that header I mentioned. I use one like this
http://www.amazon.com/PC-Internal-Mini-Onboard-Speaker/dp/B002W4M0DW