Are you absolutely positive Is the card is the retail X1900 G5 Mac Edition and not an X1900XT pulled from a Mac Pro? Because the latter won't work in a G5 and the former doesn't have the letters "XT" in its name...

(On the reliability question, the X1900XT in my Mac Pro still worked fine last I checked, but I haven't used the system much the last two or three years. And yeah, it runs hot, just like the rest of a 2006 Mac Pro.)