Is it possible to change behaviour of Fallout 3 ? I would like to modify the the RPG combat system to play like a FPS. Is there a mod or something to do for that ?
Take a look at Fallout Wanderer's Edition. It's probably the best regarded overhaul for the game, and should be tweakable to get whatever you happen to define as 'playing like an FPS'. If FWE doesn't float your boat, the Nexus is home to other overhauls that might suit your needs better, but FWE is the big one, and it's the one I'm familiar with.
Oh, and don't use VATS, but that goes without saying.