i386 runs on 80386 and faster computers. 25-100MHz processors
i586 runs on pentium I and faster computers. 60-200MHz processors
i686 runs on pentium II and faster computers. 133-350+MHz processors
Not really that much of a distinction, it's just that the higher ones are optimized to take advantage of the newer systems. Most people with systems newer than 1998 will use the i686 rpms. Worst case scenario, if you try and install them on an older system, you'll get an error message. If anything, they need to bump some of these settings up a little nowadays, as i686 only went to 600Mhz even in server systems.