Do not buy Icelandic currency in your country, as you will get a very bad rate. There are banks and Cash machines at the airport and they are in service 24 hours. Also Visa and MasterCard are accepted everywhere in Iceland and there is no minimum.
You can pay taxis and buses with a card and even buy a cup of coffee or a bottle of Coke with a credit card. Amex is not widely accepted as it has been badly serviced here in the past years but Visa and MasterCard are accepted everywhere.
The current exchange rate for selling euro is about 87 Iskr and US $ is about 65 Iskr and 128 Iskr for each GBP. This can fluctuate. Exchange rate can be found at www.landsbanki.is .