I picked up Fallout 1 and 2, and gave them both a try. Fallout 1 really didn't seem to have nearly as much choice as people suggested. There might have been two or three situations i could have talked someone out of fighting, other than that it was max energy weapon skill and run around disentegrating crap in Power Armor.
Fallout 2, it took me a month and a half of on again off again playing to get past the horrible beginning. It seemed to have a little more in the way of choice, but not a whole lot. Again, max energy weapons, power armor, stuff goes poof.
Fallout 3. The only choices you can really make that leaves any lasting marks on the world are a

Destroy Megaton and b

Destroy the Citadel from Adams AFB in broken steel. Both Jerk moves(and one is completely retarded) but I'm ok with that. The rest is pretty linear. However I probably spent something along the lines of 300-500 hours playing Fallout 3, maybe 12 in Fallout 1, and 20 in Fallout 2. More to see, more to do, 3 is just an all around better game.
Edit: By linear, i mean it's going to happen whether you like it or not. Like you can either destroy Eden, or when you wake up in Broken Steel Liberty Prime did it for you. Nothing really leaves any marks outside the norm aside from the above stuff