You should take out about 22 times as many Korunas and you would USDs.
Credit cards are welcome in most places. I have always gotten a very good exchange rate when using my Card. Small businesses really appreciate cash as they don't have to pay transaction costs. As a visitor I like to keep the locals happy.
Five days, try a hundred USD, 2200CK. Do check around as to which ATM gives the best deal. When you arrive in the airport, you could look to see which ATM has a line. Or once you in Prague watch for usage. Don't be like me on my first trip to Ireland. There were two Fish and Chip shops, almost side by each. One had a line and I went to the other because I didn't want to wait. Oops.
wayne iNWI