Let me break it down for you:
proftpd-1.2.8rc1-1.i386.rpm
The "rc-1" means release candidate 1 .. in other words, this is a beta version of the code. Not meant for server usage.
proftpd-inetd-1.2.7-1.i386.rpm
The "inetd" means that this package is meant to run behind an inetd "master". This is good for newboe admins who want to use inetd to control services + security.
proftpd-standalone-1.2.8rc2-1.i386.rpm
The standalone version is what you should use if you are running any kind of server. This however requires a good grasp of the linux operating system.
I've noticed that all of the "rpms" you have posted are compiled for i386 .. and they are not even mandrake specific. Go to
http://www.rpmfind.net and search for proftpd. When the list comes back, type "mandrake" in the box labeled "system" and then click on "search" again.
This will search for all proftpd rpm packages and then narrow it down to only mandrake specific rpms. If you use mandrake, try to stick to mandrake specific rpms - they are compiled for the i586 archetecture and will generally run faster and more compatible with your install of Mandrake 9.1
PS: proftpd-1.2.8-4mdk.i586.rpm is what you will want to install.
When you download the RPM package, use: " rpm -Uvh proftpd-1.2.8-4mdk.i586.rpm " to "upgrade" to the latest version.
Hope this helps