I wouldn't chance it. The 750 is a 55W card, and the 9550 is a 95W CPU. Your 12V rail is rated at 13A, so it can supply, in theory, 156W. The CPU and GPU alone would consume 150W. This leaves basically nothing for everything else. And don't forget that not every PSU is truthful, nor do they last forever. There is a good chance due to the age of the unit it might be on its way out. I would replace it before adding a GPU. It just doesn't have a lot of power.
In theory yes, but its better to be safe rather than a new headline breaking news due to fire and a total loss of a perfect good pc.
would this be enough for them?