Go Ubuntu! Use an LTS release, 12.04 if you need it up and running next week if not 14.04 LTS comes out soon, April 17/18th I believe. You will be fine with that hardware even at 100 concurrent processes which could be anywhere from 200-1000 users currently online.
Tune Apache < If you need help just post here or PM me!
Tune MySQL < If you need help just post here or PM me!