Hi,
As said bellow the thread, use benchmarking tools. I am authentication 15k users in AD (3 times as much of you), Vmware VM. 3-5k daily users and 512-800MB of RAM is more than enough for me for running FreeRadius (in fact in memory the binaries only use 70MB). The detail accounting only grows in the tune of 3GB per month. The MySQL database is on another server. As for the MySQL server you may need more 1-2GB, so 4GB is more than enough for a dedicated server without bloat and FreeRadius + MySQL). My CPU requirements are also *very* low, two CPUs load average: 0.00, 0.01, 0.05.
I'd agree (though we use postgresql for better performance ;-) ) - however I'm not going to make ANY recommendations off the hoof on this list due to future implications on it - such as 'you said it would work with XXXX but its not so I'm suing you' :\ alan