For information about AIME, the Art of Problem Solving Wiki is useful. You're getting into pretty serious territory taking AIME in 8th grade. If you look at their general statistics for 2008, you'll see that 332 8th graders participated. Their scores varied from 0 to 12 out of 15.

They do two AIME exams each year. My DS12 will be taking the second one next week. I'm hoping he does well enough to get to USAMO, but that will surprise me.

Regards,
Jon