Respective bankroll for certian limits in NL holdem

I have been playin online poker for a few years. I started out playing the 25nl(max buy in) game and have now progressed to feeling comfortable playing in the 200nl game as far a my ability is concered. But i never have the correct bankroll to be playing this higher limit realistically. I always find myself winning big and cashing out. Or going bust on my initial buy in. I would like to be a more consistant player(i.e. not busting out and building a large online bankroll.
I was just wondering, in terms of how much money is in my account, what limit should i play. And when is it a good idea to move up in limits in terms # of buy-ins at that level. I am willing to start over at the 25nl game and work my way up, hopefully to the very large games. Also, what should my initial buy-in be when starting out at the 25nl game.