I recommend you hire a security professional or consultancy to review and secure your system. If you wish to handle it on your own then I would recommend a number of books, including Tony Mobily's excellent Hardening Apache and my own book on Hardening Linux.
But I strongly urge you to engage a professional. Your ISP/data center may already use someone or an organization that might be able to assist you.
This was first published in January 2006