Every time I play New Vegas or Fallout 3, after about 30 minutes, the game randomly crashes to the desktop with no error messages. Is this a game problem or is it my graphics card overheating, or is it something else? Please help!!

I loled at the title
One thing that helped me is you can set the game to force launch dx9 through steam by adding "-dx9"(minus the quotes) in the launching options.
Id double check it for you but Im at work right now

If your video card was overheating your screen would go black and your PC might shut down. If it was a video driver crash it would notify you of that. Its most likely just a game crash.