I think you've wandered into the wrong section of the forums
However, I shall try to answer your questions
Many "300W" power supplies are actually not as good as they say they are. I would believe that the 250W in the Shuttle is of a much higher quality than most "300W" power supplies you can get. Matrox probably says that to stop people putting a card in a PC with a low quality power supply. With the enthusiast market the Shuttle is aimed for, putting anything less than a very high quality power supply would be like shooting yourself in the foot. You can still move, but it isn't really what you want!
I imagine that it should be OK running on that power supply.
I do not own a Parhelia, so I cannot comment on the volume of it. However some people have different perspectives of what is "loud"
To some, if you can hear it at all, it's loud, but to some it has to be screaming "I'M A FAN, YOU MUST BE ABLE TO HEAR ME!!!!" for it to be considered to be loud!