Some people say 30-40 buy in's, others up to 100. Personally, i like having around 50. So if your playing with a $300 bankroll, then you should be ok playing the $5+$6 SnG's, similar (perhaps slightly less amounts- 100 buy ins) in MTT's and should be sitting down at ring games with around the same again. Keep this in mind and step up a level when BR allows, so if you hit $500, you should be ok stepping into $10 games. If you drop to <$150 then drop to the $2 games etc.
Dont move up a level just because you have the cash either, test the water and see if your comfortable at the new level first, you might feel you'd be happier with 100 buy in's at $20 as there are less bad players, so the swings may be moe severe in winning/losing streaks. Do allow yourself odd forays into higher levels from time to time. I play mostly $5 and $10 now, previously $2's, but i've had a crack at the $24 and $75 games on full tilt using the 'token' games, and have played higher games through satellites. Experiment.
I'd also lean towards being nitty with your bankroll, most successful players are.