10 - 12 "zone"/client computers? Are you setting up a video-viewing shop?
The critical aspect will be your server's ability to service so many clients concurrently, with a worse case scenario of all of them wanting to access the same video or audio file/track.
Your client computers configuration can be standard; Media Centre is essentially XP Pro with extra GUI modules integrated, so either will do.
I don't know what exactly you would want such a setup for, but I can't see what extra benefits dual-boot capability will give your client PCs.
It depends what level of service you intend to provide each client computer ~ the optimum would be the ability to have separate partitions with server based software handling the load balancing and masses of memory so particular files which are in high demand are left resident in memory.
As you are struggling with XP/Media Centre for the client PCs already, the server side is going to be a lot more complex (and costly).
There simply will not be enough space even in this forum, to reasonably "offer all advice possible".