PC crashes like that usually occur when the user has been playing for many long hours without a break and the GPU falls over on exit. You may see a slight lag before exiting the game which is an indication that the GPU ram is almost totally spent and not much left to include the desktop on exit. You might get to the desktop but no task bar or icons appear (or take a long time to show).
Make sure the hdd has more than 20% free space as game play uses much of the free space for temporary files.
No, you can't plug in a second PSU.
The GTX460 are not exactly the top of the range and would have limited ram capacity and speed.
Basically, save when ever you can (single user mode) and don't play for more than 4 hours continuous play.