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I replied to the PM. Most of it I'll include here for clarity. Some of these things you've already done and are seeing improvements.
If you don't do a lot of perl I would say disable mod_perl since it can eat a lot of memory.
I don't know all of the modules but you might consider if you need: dba, dbx, hyperwave, etc.
For security I would turn register_globals Off and only enable for specific modules that really need it On (.htaccess or in httpd.conf)
One setting I've used to get a bit more is in httpd.conf:
<Location />
# Insert filter
SetOutputFilter DEFLATE
# Netscape 4.x has some problems...
BrowserMatch ^Mozilla/4 gzip-only-text/html
# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4.0[678] no-gzip
# MSIE masquerades as Netscape, but it is fine
# BrowserMatch bMSIE !no-gzip !gzip-only-text/html
# NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
# the above regex won't work. You can use the following
# workaround to get the desired effect:
BrowserMatch bMSI[E] !no-gzip !gzip-only-text/html
# Don't compress images
SetEnvIfNoCase Request_URI
.(?:gif|jpe?g|png)$ no-gzip dont-vary
# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary
</Location>
this adds compression on top of what Zend will do.
When running top is httpd always at the top with long CPU times and using most of the resources?
Make sure you don't have things like DNS resolution in the web logs (isn't by default).
If you have multiple disks you could split some of the IO over the drives. The temp space is one, swap is another, web files vs db files are best over unique disk channels.
Check if you are quickly going into swap space. As much RAM as you can get is always helpful.
Maybe more tweaks.
Oh, .8 and 80 queries is a tad high. Try to get the queries down and it will speed up some more.
