Namibia uses the Namibian Dollar (N$) this is linked on a one to one exchange with the South African Rand. The Rand is legal tender in Namibia. But the N$ can not be used in South Africa. N$ can also be changed to Rand without commission in Namibia, but the same transaction in South Africa will attract a fairly hefty commission..
If you are wishing to purchase currency before arriving in Namibia it is easiest to buy Rand as the Namibian Dollar is seldom available in banks outside of Namibia.
To summarize visit Namibia with Rand in your pocket, and make sure you exchange any N$ you have back to Rand before you leave the country.
Credit cards are accepted at most supermarkets, restaurants and accommodation establishments in urban Namibia (Visa and Mastercard are most commonly accepted) but outlets in rural areas are more likely to only accept cash payment.
Banks can exchange travellers cheques into N$ but this will attract commission, only a few establishments accept travellers cheques, so its best to cash some travellers cheques whenever you find a bank. Larger Namibian Hotels will exchange money, but rates are generally not favourable.
Banks are open from 9am-4pm on weekdays and from 9am-11pm on Saturdays and closed on Sundays.
"I believe that banking institutions are more dangerous to our liberties than standing armies. If the American people ever allow private banks to control the issue of their currency, first by inflation, then by deflation, the banks and corporations that will grow up around [the banks] will deprive the people of all property until their children wake-up homeless on the continent their fathers conquered. The issuing power should be taken from the banks and restored to the people, to whom it properly belongs." Thomas Jefferson