iNumHWThreads=
Set it to the number of threads your system has. For instance, if you have a 2 core i3, set it to 2. 2 core hyperthreaded i5, set it to 4
I read it can only go up to 4, but I have mine set to 8 (4 core hyperthreaded i7) and have played for four hours without the freeze. Once again, this is a freeze in gameplay where the music still plays and you cannot alt-tab out but can cntrl-alt-del and shut down fallout3.exe from the task manager
I'll update this if it crashes, but so far so good. This appears to limit the number of threads the game tries to use. Just because you have a single core processor does not mean the game only prepares 1 thread, it still prepares multiple threads as do all applications really. I have noticed no decrease in performance either