What grades will I need to study at a good college in the USA? I'm a British student?

Currently studying A-levels and I am targeted 3 A's, in Politics, History and Geography, I am going to take my SAT's sometime in Autumn, and I should do reasonably well. I know it's difficult for British students to get in to American unis but do you think I have a chance at places like, Cornell, Columbia, Brown etc.