You should play at a level that you are comfortable at and that your bankroll can handle.
Minimum banroll for limit games : 200-300 big bets (buy in for 25 big bets at a time)
Minimum bankroll for NL games: 20 buy-ins (each buy in is maximum, so at a .05/.10 table, a buy-in would be $10 -> $200 bankroll)
Minimum bankroll for SNGs: 30 buy-ins
MTT Bankroll: DOn't spend more than 5% of your bankroll on one tournament.
These are general guidelines and ensure a safe bankroll that will last a long time...you can definately play below these standards, but make sure you don't risk too much (one bad beat and you are broke for good).