I have just installed Centos 5.5 by using the Netinstall version. Seems that on my x64_86 system I have a lot of i386 packages installed. To remove them I had to :

yum -C list installed | grep i386 | awk ‘{print $1}’ | xargs yum remove -y

Also edit /etc/yum.conf and add :

exclude=*.i386 *.i486 *.i586 *.i686 *.athlon

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