that's wonderful that you're thinking of studying abroad at yonsei.
i went spring 2007 and stayed in I-House.
i'm also korean so i stayed an extra month at my aunt's house. well, initially i was going to look for a hasook jib for that one month during summer but that costs money (maybe around $300 at the low end and probably no AC at that price. killer) and time (instead of looking around for a place i decided to spend more time with all the people i met there since i'll never see them again haha).
as for improving your korean, that is something entirely up to.
i specifically went for that purpose cuz my korean isn't too great and i'm also a bit embarrassed that i can't speak it fluently.
i picked up a few more words, learned some hanmoon, spoke a lot more fluently whenever i called back home to my parents. but when i came back i think i actually got worse haha.
my roommate who was chinese actually improved her chinese more than her korean cuz most of her friends spoke chinese.
i partially blame the layout of the campus cuz most if not all your classes will be on the east side where all the international studies and I-House are. you'll have to trek over to main campus if you want to encounter non-foreigners.
but mostly i blame myself cuz i didn't make that much of an effort to take classes on main campus and meet korean students.
there's clubs (yonsei international community & mentors club) that'll reach out to you so make sure to take advantage of that and keep in contact with your "buddy" that they're supposed to pair you up with.
there's also language exchange where you meet up with a korean to practice korean for you, english for them. it's not always necessarily a formal thing but you can just set it up with anyone. people post up ads in global lounge which is on main campus.
if you want to play late (bars, norebang, clubs, etc) then I-House is most convenient since most of your friends will probably live there. the disadvantage of living there is that everyone speaks english. you ahve to really make an effort to keep using korean. and being in a foreign place for so long can make you want to only speak english. you ahve to try to step out of your comfort zone haha that sounds really corny but it's true. don't be afraid to make mistakes. plus you took 2 years of korean which is plenty to get around.
hasookjib is good but you usually have to be quiet and sometimes they don't allow guests. rooms are also the size of a closet.
you could look for an officetel (오피스텔) which is more like an apartment, but it costs more.
there's deposits for officetels and probably for hasook jibs too. can't remember.
since you're going for 2 semesters you might as well try out I-House and then move out if you don't like it.
i'm sure you've heard of the 2 language programs. KLI and DIEE. KLI is 4 hrs 9am-1pm. DIEE is 2 hrs 4pm-6pm. you usually have to sign up way early for KLI and they have a different schedule. it kinda sucks for people who have 9am class cuz they can't stay out too late. but if you're a morning person and want to really learn korean, hey, KLI's for you.
EAP's a good program from what i've heard. i went thru ISEP. EAP students got to go to japan which i was really jealous of until i went later on in the summer

AND randomly met ss501 haha anyway...
i mainly went thru ISEP cuz it was the only one at my school that had other korean universities like korea university. but everyone kept telling me seoul u has all teh geniuses, korea u has the country people, and yonsei u has the cool kids. but that's just something i hear from basically every korean parent. haha. i was still willing to go to korea u cuz i wasn't going to play. i wanted to study korean. but i ended up at yonsei. and played... oops
staff are usually nice. the program coordinator for the states changed so i don't have info on that. there's one really nice 경비아저씨 (security guard) for I-House but the other ones are kidna weird.
you'll meet so many people and learn a lot while in korea. i'm really jealous and hope to go back after i graduate too (i can't remember who posted that). whichever school you go to will be fun. yonsei might actually be your best choice for korea. in the fall, they have YonGoJun =
연세대 +
고려대 or GoYonJun in korea u. sports teams play against each other. akaraka is basically a rally takes place in the spring and it's to get ready for YonGoJun.
wow that is wayy to long of a post. sorry. i hope i helped. if you have any other questions you can email me. p2bandj@yahoo.com
have fun~