​postgresql-server is in the updates repo

The repo http://vault.centos.org/5.7/updates/x86_64/RPMS/​ contains
postgresql84-server-8.4.9-1.el5_7.1.x86_64.rpm

This will add a repo
# cat > /etc/yum.repos.d/centos57-updates.repo <<EOF
[updates]
name=Updates
baseurl=http://vault.centos.org/5.7/updates/\$basearch/
gpgcheck=0
<<EOF

Then install postgresql84-server

# yum makecache
# yum install postgresql84