I have an AMD 3.1 tri-core, 8 gigs of ram and a Geforce 250 gfx card. I just got a second monitor and since hooking it up all of my games crash on launch. I've tried resetting the screen resolution and installed the dx9 dll. What happened?

If all of your games crash, then at least this isn't a FNV bug.
Also, I've seen a note about the dll saying that it might crash with a second monitor.
You might want to check if you have the correct driver, just in case.
PS: I've got a dual screen setup myself at the moment and it is working just fine with or without the dll.