In Seoul, you can use your Visa card everywhere. To withdraw cash, you must find a Global ATM, but they're usually at every bank. Also, at the convenience stores, most of the ATMs that have an English option will take your foreign card. At a bank with several ATMs, you just have to make sure it's a Global ATM. There will be a sticker on the machine.

Japan is a different beast for ATMs. Very few are global. I remember having difficulty there at several banks.