Does anyone have any evidence or stories regarding running a Redhat FC system dedicated as an email/SPAM filter on 1 proc vs. multiple procs, where the multi-CPU system has much slower throughput?

I currently run a Sendmail 8.12.8-9.90 system, with the following tools installed:

MIME-Base64-2.21
Digest-SHA1-2.06 mimedefang-2.39 HTML-Parser-3.34
Mail-Audit-2.1 MIME-tools-5.411a-RP-Patched-02
HTML-Tagset-3.03 Mail-SpamAssassin-2.60 IO-stringy-1.212
Mail-SpamAssassin-2.63 MailTools-1.60
sendmail-8.12.10 libnet-1.17 MIME-Base64-2.11
Time-HiRes-1.52

We run this as our border email SPAM/Virus filter (Sophos AV).

Since my current 1 Intel P4 2.4ghz machine is contantly closing the IMC due to having 12 threads of execution busy, would it be better to move to an older server that has older (pIII), slower (500mhz) processors, but still 3 or 4 CPUs, or will I not see much of an improvement or even hurt my performance in this type of application environment?

Any comments that anyone has would be appreciated

Keith