SpamAssassian with FreeBSD and Big Mail Server

[Vahric MUHTARYAN]

Hi Everybody , 
 
            Really I don't know can I say a big mail server which have
30,000 mailbox on it 1200+ simultaneously connections (pop,smtp,webmail).
İncoming smtp connections are between 200-400 . We want to run spam software
on it but machine can't handle it for this reason we seperated machine
freebsd+exim+SpamAssassian but on 400 connection machine goes down average
is very high , cpu usage really too high . 
 
            I want to learn Anybody Who have closer or bigger system and
using SpamAssassian ?! 
Really this 400 connection simultaneously can be limit for spam software ?!
I mean Anybody can handle more ?! 
I have to design distributed environment ?! 
 
My Hardware is (for spam)
            2 X PIII 1G +  1 GB RAM + 2 DISK RAID 0 SCSI 10000 RPM   
 
Thanks
Vahric