That's probably why you can't see your temperatures then. Because your card doesn't have a sensor to read the temp on it. Hmm, try opening the case and plonking (placing) a large desk fan pointing directly on top of the graphics card and CPU. Run your system with the desk fan blowing into the computer and see if it locks up again. By the sounds of it though I wouldn't have thought it would be a temp problem especially with it doing it so early into the boot up. Sounds more like a hardware issue somewhere. Did you flash the BIOS before (or has someone else for you)? If not, and you haven't had this issue since having the machine I'm going to guess it's not BIOS related. More hardware failure than anything.
:: Edit ::
The HIS Excalibur edition of the X1950 Pro is a very nice card. Runs smooth and barely hits over 45C even under heavy load. A little large in size though within the case and cable management around it is impossible lol. Performance wise the card seems very good yet I don't get quite as much FPS out of it in game as I would like. I get around 130FPS on CS Stress Test with everything on high including the filtering on Max so I'm not sure if thats good for that card or not.
Breaks.