Correct Approach to Architect a Portal

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Large Company has approx 1400 websites, web applications, and web portals. They would like have a single entry point Corporate Portal with integration to the existing 1400. The main platforms that these 1400 web apps are developed with and running on are Microsoft .Net and J2EE Application servers (Sun and BEA). What is the best approach to start tackling this opportunity. Should one go as far as defining an Enterprise Architecture using a known framework such as TOGAF, etc. and then search for and select a portal architecture framework, platform and product? Or, should one immediately start with working on a portal architecture? Any ideas around this topic will be appreciated.