In reality, is your northbridge fan running???
The 9800 pro should get pretty damn hot, and is probably located right next to your NB chip - if that fan is dead, and you're starting a game... oops.
You can check the temperature of the videocard by putting your finger (gently!) on the back of the card, where the GPU would be. If you can keep your finger there for longer than a few seconds without burning it, the card's fine.
Also, try leaving the case open, see how that works. If the reboots occur later, that might be an indication of temperature troubles.