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With that sort of load I expect you'd have a dedicated server? I think what you want can be done with Xoops, but only with careful consideration and planning. It would be impossible for anyone here to say if using XOOPS would or wouldn't present a security risk, as apart from XOOPS itself you also have to consider PHP, MySQL and your webserver. There is no "non-PHP mode" as such, and it would take some work to get your system so that it only serves static content to your users with XOOPS operating as the CMS in be backend. Don't know what your requirements or website entails, but I would almost say you are better off with a fully customised application built to your needs than traying to mould something like XOOPS to do the job. That's just my initial impression tough and I only rank myself as an advanced XOOPS user at best. Guys in the core design team such as Mith or Skalpa would be better suited to answer such a question.