could my beyond the sword multiplayer play wiht a warlord version of civ4?

also, can a PC link with a mac?

As far as I know, you have to play the same expansion of Civ4 at both ends of a multi-player game. You may be able to get away with different versions of an expansion, but the software will warn you that they are not the same.

Yes, a PC can link with a Mac, either locally or over the Internet. However, you will be limited to vanilla Civ4 version 1.74 and Warlords version 2.08, which are the latest versions available for the Mac. Also, there have been reports of synchronisation problems.