Your computer is simply not powerful enough. Even though you might meet the bare minimum specs minimum specs are just that, the minimum required to run. No game ever ran smoothly with minimum specs. Fallout 3 came out at the end of 2008 and was a pretty demanding game at the time. I either had an 8800 GTS 320 or a 9800GTX+ when it was released and I don't think I could max it then.
System requirements for the game.
http://www.game-debate.com/games/index.php?g_id=25&game=Fallout%203
Graphics card hierarchy chart.
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html