If you want to maintain your high opinion of Fallout 3, I'd warn against it.
Hahaha... that's funny... not really true, but funny.
Anyway, as others said they are very different games. FO3 is an action/RPG hybrid with FPS gaming, and the old games are pure RPGs with isometric grid-based gameplay and turn-based combat... it's like the different between Oblivion and Final Fantasy, in size.
They are great games though, for sure. They are so cheap to get I would advise any RPG fan get them, really...