Do you have the 4 pin 12V power attached as well? It looks like half an AT power supply and goes on as well as the standard ATX one. I think you need a "Pentium 4" power supply to have this connector.
That's all I can think of at the moment and it's taken from the link to the Asus website. You said that whatever is in that link "might be a factor" - is this what you meant?
Sorry if it's stating the obvious a bit.
Here are a few other things you could try: make sure the memory is in firmly, and just try it with the most basic components; mobo, cpu &fan, memory, graphics card, keyboard & monitor.
I've discovered on this forum that it's not necessarily just the wattage of the PSU, but its efficiency, although I doubt your 300W one is a P4 power supply.