IIRC
You Choose between House and Legion when you goto the Bunker at Legion Fort.
If you destroy the bunker = Legion
If you don't = House
Not sure where NCR and House divide happen, it might be with the Boomers, but I think it has to do with following the Ambassador's route.
[Edit]
Wild Card is you becoming your own power in the Mojave. By killing House and assuming control of his robotic army and killing/converting the minor factions to your side.
When I went to the fort I wasn't really prepeared for it. I ended up want to bust Benny out (strange that when he was the one shooting you in the head to start with).
But with no pip boy to give him, the only option was to untie him. Of course that wasn't fun with only a pocket knife to show for (of sorts).
Needless to say, after a lot of frustration, I left the fort as a smoking ruin with dead bodies piled up everywhere, Benny running off, and what happens next, no idea....
As for the boomers, well when got the choice to ask if they wanted to support me/ncr, I went for the "me" choice. and for the ambassador I believe he said something about it was good they was supporting "us" and the response you could do was "me" something..
I'm just basicaly stomping around, chewing bubble gum and blowing stuff up. (and do what feels right). I guess that could wreck some quests here and there...