To upgrade to CentOS-3.4
------------------------

rpm -ivh http://mirror.centos.org/centos/3.4/os/i386/RedHat/RPMS/centos-yumcache-3.1-0.20050105.3.noarch.rpm

(to avoid the header downloads)

then

rpm -Fvh http://mirror.centos.org/centos/3.4/os/i386/RedHat/RPMS/centos-release-3-4.2.i386.rpm

to get the new releasever

then 

yum update