Spam/Antivirus Options for Sendmail

This is kind of a broad question so bear with me.  i'm basically setting up a relay server on our network for our internal mail and for client mail (clients pay us to relay for them but the box they are on right now is kind of...unstable).  We went out and bought a dell dual proc 1400 intel and redhat enterprise 3, and i've already got it installed and sendmail is working fine (from the testing i've done so far).  now one of the reasons we broke local mailboxes off from the relaying component is because of load issues when running high end mail filtering like spam assassin and antivirus.  at one point before i worked here i guess they tried that on the old box and it caused the queue to jump up a few hundred emails every 15 minutes.  now i've been tasked with this job of setting up a relay and while i know a little linux, i'm not as high end as the guy who used to work here.  I've been looking at filtering systems for linux and there is alot out there that seems to overlap other products. the ones i'm looking at right now are Spamassasin for spam (and spamassmilter for plugging it into sendmail for relay i guess?), clamav for antivirus, mailscanner (not sure where it covers but people keep telling me good things about it) and a sanitizer.pl that is running on our current box.

The question is, do all of these products compliment each other or do some overlap and are not required?  would i install them all individually or does one install peices of another one?  i'm basically trying to figure out what to put on this box before we move it to production so those people we relay for can have antivirus protection (if possible) and spam protection (at the bare minimum, spamassassin) and anything else that would be a bonus to the client.

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