I've been helping another guy here on yahoo figure out similar situation as yourself.
Is your GPA on a 4.0 or 4.5 scale? Regardless, a 4.2 is very high. But your SAT score is on the average-low end. If you can take your SAT again, I'd aim for a 800 math (very doable with 1 month of practice) and 500-550 on the verbal and writing which will bump you up to 1800-1900 range. Your extra curriculars won't be weighed as much as they may like in the states.
As an overseas Korean, competition is going to be rough, especially for business school. On average, you're looking at 30-40:1 competition. Korean schools will usually accept 2% of their students as overseas Koreans. This usually equates to around 1-3 people per major. It essentially means that of all the applicants of a certain major, the ones with the highest gpa/sat score will usually be the ones accepted. If you can raise your SAT score a little, you should be alright for the first wave of acceptances. You might even be fine as it is, as your GPA is probably on the higher end. FYI, the first wave is determined solely by your documents (gpa, sat, ap tests, personal statement etc)
As you may have already figured out, overseas Koreans will have to take an exam (English, math, Korean) if they pass the first wave of acceptance. If you get to this stage, you MUST do extremely well on this test. From here, you are accepted in the order you score. The 1-3 people with the highest score WILL be the ones accepted. There are hakwons that prepare you for these exam, and I strongly suggest you enroll as soon as you can. I've read an article that some spend 10 hours a day preparing for these tests. Given you have to score the HIGHEST, I am not surprised.
Here's some data from 홍대. Keep in mind that this is a middle tiered school in Seoul and that Yonsei and Korea will probably have higher competition.
http://ibsi.hongik.ac.kr/2011files/20100805/2011for.html