I've been having problems with Fallout 3 CTDs as well, and wanted to add my experience to the knowledge base here...
I was having major problems in the Mall area of the game - random crashes both on the Mall and in buildings leading to the Mall, and guaranteed crashes when trying to use VATS. I was using older nVidia drivers - 178.xx - so I upgraded to the most recent ones. Still had problems.
Tried ffdshow fixes, upgraded the game itself, tried everything else mentioned on this forum...nothing.
So I rolled back to nVidia 180.48 drivers, and haven't had a problem since.
So, for some reason, both older and newer drivers were crashing, but that 180.48 has been stable.
My rig:
2.8Ghz Core Two Duo
GeForce 8800 GTS 512
4GB Patriot RAM
Windows Vista Ultimate