How many domains are you running?

[raphael]

In one of our servers we are running 104 websites (not counting subdomains).

Quote: Dunno how they manage .. my mail server has to be a server on its own, spamassasin en antivrus are consuming too much cpu ..

If you are using clamav-milter I recommend you to make it use clamd instead of starting an other instance (and make services use clamdscan as possible). An other way to reduce load is by making queues instead of starting more processes all around to process everything at the same time.