proper way to yum update i386 on x86_64

[Jack Howarth]

    I have been installing specific i386 packages (such as Firefox)
on machines with x86_64 Fedora Core 4 using the following approach.
I created fedora.i386.repo, fedora-updates.i386.repo, and 
fedora-extras.i386.repo in /etc/yum.repos.d with contents of...

[base-i386]
name=Fedora Core $releasever - i386 - Released Updates
baseurl=http://download.fedora.redhat.com/pub/fedora/linux/core/4/i386/os/
#mirrorlist=http://fedora.redhat.com/download/mirrors/fedora-core-$releasever
enabled=0
gpgcheck=1

[updates-released-i386]
name=Fedora Core $releasever - i386 - Released Updates
baseurl=http://download.fedora.redhat.com/pub/fedora/linux/core/updates/$releasever/i386/
#mirrorlist=http://fedora.redhat.com/download/mirrors/updates-released-fc$releasever
enabled=0
gpgcheck=1

[extras-i386]
name=Fedora Extras $releasever - i386
baseurl=http://download.fedora.redhat.com/pub/fedora/linux/extras/$releasever/i386/
#mirrorlist=http://fedora.redhat.com/download/mirrors/fedora-extras-$releasever
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-extras
gpgcheck=1

These work fine for installing specific i386 packages with yum using the
--enablerepo flag set to the appropriate repository. However I am quite
unclear on how I can cleanly use 'yum update' to update only those i386
packages installed to the latest versions without accidentally installing
additional i386 packages for other packages that are already installed
as the x86_64 version only. Thanks in advance for any clarifications on
this issue.
            Jack