Tero said:
Cash games require larger bankroll than tourneys do. That is the general rule.
Sorry to say, but this piece of information is not correct.
Due to the higher variance of tournament poker you need a far bigger bankroll for playing tournaments than for playing cash game.
Bankrollmanagement rules of thumb:
Considered bankrolls for middle conservative BRM:
Cashgame 20 - 50 Buy ins (not very conservative to more conservative)
NL50 with these Ranges 20 X $50 to 50x $50 = $1000 to $2500 needed bankroll
Tournament 100 - 300 (not very conservative to conservative approach)
Buy in of $20 MTT -> 100 x $20 to 300 x $20 = $2000 to $6000 needed bankroll
Also check primedope (google it) for Bankroll calculators, there is a nice risk of ruin calculation there and an interesting graphical part.
I would have answered in the correct forum section but answer to OP was given here.