Worth getting an Oyster Card?

I travel to London about once every 2 months for day trips as I love the City and I use zones 1, 2, 3, and would like to explore the DLR and overground lines and zone 5 and 6 in the near future, I pay the �8 for a daily paper travel card but would getting an oyster card benefit me? and what is the minimum amount of money I can put on it and how much is it for the card?