Recommended hardware spec 20000 users

[Neil Wilson]

Hi guys,

I need some advice please,

What are your recommendations for hardware for a server that does Spam 
scanning with SA, Clamav scanning, running postfix under Centos 5.4 and 
logs in mysql and then forwards the emails on to an Exchange server for 
+-20000 users?

The emails theoretically have some basic spam scanning done before they 
get to the Linux server however there will still be a large amount of 
junk email to get rid of before passing it on to Exchange.

There is currently a dual proc dual core Xeon server with 512MB of RAM 
running amavisd, clamd and postfix handling the email at the moment, but 
this is very outdated and lacking functionality so it needs to be 
upgraded asap.

Would the current server handle the load with just a large RAM upgrade, 
EG: 4-8Gigs?

Any advice will be appreciated.

Thanks.

Regards.

Neil Wilson.