1) How much RAM is necessary for 100 students simultaneously? Would 4GB of RAM be overkill? There will be students taking quizzes that involve audio.
Check this http://moodle.org/mod/forum/discuss.php?d=72766
2) What would be a good processor speed for 100 students?
If you buy a state of the art server, in the case of Moodle, RAM is the bottle neck. Get Q1 settled.
3) Is ubuntu the most efficient OS to use?
Ubuntu is not an OS, you mean Linux. Ubuntu is perhaps the "most user friendly" Linux flavour. Whether that is an advantage, you should know.