I think what you're asking is how to download the correct server release from Minecraft, because the download page always lists (just) the current version.
Fortunately, the download link is consistent. The current link is:
So, you just alter the versions a bit, and you've got it. For 1.7.2 it would simply be:
And for 1.6.4:
As others mentioned, you need only to modify your client profile to indicate which version you play on your end, and then you'd install the mods in whatever way your mod loader needs them to be installed.
I'm pretty sure what you're actually after is the server link, so yeah - that should do it.