[CentOS] question about network repositories

Hi

I'm thinking about setting up a local / LAN respository for my CentOS, and probably other (Fedora Core) disto's, but have never done it before, so here's some questions:

If I setup a repository, can it hold different distro's / architectures & versions? Say for example, CentOS 4 (i386 & x64), CentOS 5.0 & 5.1 (i386 & x64) - will each one have it's own folder? Will it also be possible to only download the packages that are needed, instead of the whole 4GB repository from the upstream provider? I have some of the CD's or DVD's for all of the distro's, so really only need a few packages now and then, and instead of getting them from the net everytime, get them from the LAN repository?

Then, how would I do this?