Matt,
I had a problem with the ppp-mppe update reporting conflicts when I tried to install the i586 rpm. The install text says something about only the i386 version coming with the original install CD, so I checked the existing installed rpm's with:
rpm -qa | grep 'ppp-mppe'
to confirm what was already there & removed it with
rpm -e ppp-mppe-modules-2.4.2-4es).
I then ran the rpm again and it worked no problems.
Of course, there's every chance that the problem arose coz I installed the wrong version some time in the past while 'experimenting' (this is a development server). Either way I got it to work in the end, which I think is what you wanted to know.