One of the best rate is the one near China Embassy... check out their rates online here...
http://www.embassyexchange.com/
remember i saw 980 at Myeongdong last week. we managed to change some cash for 978 there next day.Last week I changed at myeongdong @970. It's true that rates are better there.
Very little cash needed as almost all places accept cards.How much to change for first timers going there?