Sharing an ADSL Line

I have a small business with about 50 users using the Internet heavy. I want to share the 6Mb ADSL connection. My present setup with the modem acting as router and modem has been very unstable the last couple of days

what is the best router to use that can handle the 50 computers plus more if there is expansion. The router must be easily configurable and must be able to limit access to the Internet via Mac Address so somebody cannot just plug in a machine and get connected to the Internet.