Anyone?

You should always try to use the latest nVidia WHQL driver, you will have many fewer problems with other games. The newest drivers may contain optimizations for some of the newer games and they may contain bug fixes so use caution when choosing to use an older driver.

However... The FNV game engine is a couple of years old. So...

The GT240 is based on a really old GPU. In just the case of FNV an older driver might run better. A couple of the very best old drivers (which were originally designed to maximize performance on the older GPUs) are v186.18 and v190.38 and possibly v197.45, try one or all and see which runs best. The v186.18 is often cited as being the fastest and best optimized nVidia driver, but it is now quite old.