I'm going to be in the same situstion as you soon, but at the moment i'm gonna take Maths, further maths, physics, chemistry and possibly biology for my A-level.
If you choose to do something about computer(like Artifical Intelligence as you said) i think you should have a good mathematical base. Have you ever thought of science engineering?
Another point is, i don't know what US university like, but in England u not only need to have a good A-level result, u also need a good CV as well to go to uni like Oxford and Cambridge, I planning to do some charity work (RSPCA, animal welfare) and some other volunteer work as well.
The most important thing is once u make the choice of what major u doing, don't give up half way.
Ping Shen