Bankroll Size
Limit Holdem
The general rule of thumb for bankroll size is around 300 times the big bet (upper limit) for limit holdem. This means if you want to play at 1/2 holdem, you should have 600$. However, you may need more or can get by with less, depending on conditions. If you plan to only play one table at a time and never play beyond the limit your bankroll is setup for, you can do just fine with 250 times the big bet. However, if you intend to play 3 or more tables at once, you should have more. You also should increase bankroll levels for shorthanded play.
No Limit Holdem
The online no limit holdem game of today is a game with a maximum buyin of usually 100 times the big blind, 10 player table. This means that at a .50/1.00 no-limit holdem game, the most anybody can initially buyin for is 100$. In games like these, the generally accepted bankroll size is around 20 full buyins. That means for a .50/1.00NL game, you should have a 2,000$ bankroll. If you prefer to play a shortstacked cash game strategy, then you might want to have between 40 and 60 buyins of 20 to 25 times the big blind. For 5 and 6 handed games, you want to have a larger bankroll because of added variance. (see Q&A section for a brief explanation)