Can you give some more details of your architecture? Are you using PHP scripts, for example? Was it a custom-written thing, or an existing script? It's really hard to give advice without knowing more about your setup -- it could be anything from Apache or MySQL being misconfigured to slow disk access to a badly-written PHP script.
How are you using the disks? Do you have them formatted as one big 24TB filesystem? It seems odd that you'd have 24 hard drives with no RAID of any sort.
How comfortable are you with Linux? Could you paste the output of the command "top -bn1 | head -n20" (Which just shows the top 20 lines of the top command in a copy-and-pastable format.)
If you're getting Internal Server Errors, you might try looking through the Apache error log. (/var/log/httpd/error_log if you haven't customized it, I think.)
If you are not really sure of what to optimise, might I suggest going to a server management company and get them to optimise the server for you. Its a small price to pay for such an expensive server you have and without any detalis of what you need, we can't help you definitely.
I HIGHLY recommend you taking a look at http://www.platinumservermanagement.com/. Then provide management services, and will optimize your server for only $29.00/per month as part of their management package.