The Namibian currency is the Namibian Dollar (NAD), which in turn is divided into 100 cents. This is equivalent to the South African Rand. There are notes of 10, 20, 50, 100 Namibian dollars in circulation. In addition, there are coins of 1 and 5 Namibian dollars, 5, 10, 20 and 50 Namibian cents. At time of writing this information (December 2012), the price of 1 euro equal to 11.14 NAD.
The currencies are still not stable. It is best to consult a financial newspaper a week before departure. On the internet you can look at www.oanda.com for example. Dollars and Euros (cash) are locally exchanged at all banks and exchange offices. The Euro is still new in Namibia, Botswana and Zimbabwe and will therefore not command such a great exchange rate.
The use of major credit/debit cards is possible in most major cities in South Africa and Namibia, both to withdraw money and to make payments. You should, however, note that ATM's can sometimes be out due too power outages or other reasons. Sometimes distances between major cities can be vast so make sure to always carry enough cash with you. It is recommended to carry cash in a money belt, hidden under the clothes! Do not take unnecessary valuables with you.