Check out my post on the subject here
http://forumserver.twoplustwo.com/showfl...=0#Post12180559
Essentially how much of a bankroll you need is based on your tolerance for risk (ROR or Risk of Ruin), the standard deviation per unit, and your win rate per unit.
I've laid the formula out in terms of BB/100 (that's what win rate and SD is based on) but it could easily be BI/tournament instead, it's unit neutral.
For MTTs this is not really so simple, I think, because the standard deviation for tournaments is wildly huge, and you'll need a pretty good sample size to even have a shot at estimating your win rate.
It's kind of amusing to play with this in excel and realize that even if you have a positive, good, win rate, you have a non-zero risk of going broke anyway. Obviously, the lower your bankroll, the more likely that is, with some small bankrolls that people use, going broke is practically a certainty, even if they're awesome.