Sure there isn't a framerate limit set in your install of FO3? Check the parameter in Fallout.ini (my documents/games/fallout 3) "iFPSClamp" it should be set to zero for unlimited.
"The game appears to have a bug - regardless of whether you enable or disable vsync ingame or in your gpu control panel, the game still forces it on by default. This is possibly why some with nice modern rigs are getting jumpy FPS and bad performance. The fix is simple: go into my documents/my games/fallout 3 open up the Fallout.ini change the line iPresentinterval=1 to iPresentinterval=0"
Worked for me super good - my FPS has now increased by 40