I see a lot of references out there talking about a "replacement d3d9.dll" for fallout 3. Some references say its only needed if you are playing Fallout 3 on an Intel GPU (which I am not, I have an NVIDIA GTX 750) but others say you need it no matter what if you are on Windows 7.

Will this d3d9.dll replacement have any benefits for me (with the GTX 750)? And if so, which of the various d3d9.dll replacements out there should I be using?

The d3d9.dll "fix" is actually a hack intended to get the game to run on Intel integrated graphics chipsets. Your video card is more than capable of running Fallout 3, so you shouldn't need this. The only exception may be if you're trying to run certain ENB packages that require a different d3d9.dll, but those should note the requirement in their install packages and point you to a safe location to download the needed .dll.

As a general rule you should never download .dll files unless they're absolutely needed and come from a verified safe source.