Should continue using CentOS 5.3?

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http://serverfault.com – Our small company has rented a remote machine some 3 year ago. The machine is used as a company server, runs our email, web, source code repository, bugtracker, forum, wiki, etc. OS is linux CentOS 5.3, with Plesk 9.5 control pannel. I think both are too old. How complicated is to update CentOS to current version? What happens with Plesk in that case? Should Plesk be updated first? Or should we continue using CentOS 5.3? (HowTos)