A few pages back, somebody posted that there's a fix for FO3 (and some other games, apparently) that tricks the game into thinking you have an nVidia GPU. From what I understand, it works with FO NV as well, but requires some additional INI changes or something.
Edit: This post is what I was referring to.
Thanks, but I don't think that's the problem...it's the mods. The game booted just fine when I ran it without them. If I try it again, I'll just need to load the mods one by one to see what's causing the problem.