yum priorities をインストールする。

yum -y install yum-priorities

　
/etc/yum.repos.d/CentOS-Base.repoを編集

vi /etc/yum.repos.d/CentOS-Base.repo

　
全てに priority=1 を追記する
(リポジトリを最優先する設定をすると言う事です)

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#
[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1
#released updates
[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1
#packages used/produced in the build but not released
[addons]
name=CentOS-$releasever - Addons
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=addons
#baseurl=http://mirror.centos.org/centos/$releasever/addons/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib
#baseurl=http://mirror.centos.org/centos/$releasever/contrib/$basearch/
gpgcheck=1
enabled=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1
[base-source]
name=CentOS-$releasever - Base Source
baseurl=http://mirror.centos.org/centos/$releasever/os/SRPMS/
gpgcheck=1
enabled=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1
[updates-source]
name=CentOS-$releasever - Updates Source
baseurl=http://mirror.centos.org/centos/$releasever/updates/SRPMS/
gpgcheck=1
enabled=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1
[extras-source]
name=CentOS-$releasever - Extras Source
baseurl=http://mirror.centos.org/centos/$releasever/extras/SRPMS/
gpgcheck=1
enabled=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
priority=1

　
RPMforgeのGPGキーをインストール

rpm --import http://dag.wieers.com/rpm/packages/RPM-GPG-KEY.dag.txt

　
RPMforgeリポジトリインストール
　当サイトはx86_64での場合です

[root@dev ~]# rpm -ivh http://apt.sw.be/redhat/el5/en/x86_64/RPMS.dag/rpmforge-release-0.5.1-1.el5.rf.x86_64.rpm
http://apt.sw.be/redhat/el5/en/x86_64/RPMS.dag/rpmforge-release-0.5.1-1.el5.rf.x86_64.rpm を取得中
準備中...                ########################################### [100%]
   1:rpmforge-release       ########################################### [100%]

　
32bit の場合は

rpm -ivh http://apt.sw.be/redhat/el5/en/i386/RPMS.dag/rpmforge-release-0.3.6-1.el5.rf.i386.rpm

　
＊RPMforgeリポジトリを無効化する際には

vi /etc/yum.repos.d/CentOS-Base.repo

にて enabled = 1 になっている場合は、 enabled = 0 にします。