[exim] Dimensioning a server

At present Ihave a P4 with 1 GB.
I receive about 40000 messages/day for about 110 users
besid exim has acteve also pop and imap server, as well squirrelmail.
e-mail processed whit clamd and spamd
Often is " suffering" and reply SMTP 451 errors due overload, or have slow
responses on reading e-mail ( about 70% user and 26% wait time at these
time)
What is the kind of machine i should think about to replace it ?