I need to support session control for 50,000 concurrent users using
MAC RADIUS authentication and up/down bandwidth throttling. For the
task I have no Dual CPU Dual core XEON 3.0GHZ/4MB (4 way).
I don't need any firewall rules, except redirecting users to an
activation portal where they can add their MAC address (no
user/password login is needed).

Can m0n0wall do the job? I read somewhere it can only use one CPU/Core.

Also I need to add CARP (but not pfsynch). How difficult is that?
I looked at pfSense too, but the current version will no do MAC auth,
will not process RADIUS bandwidth reply attributes,  and I do need to
shape upload too, which  ALTQ wont.

/John