Plex is XBMC. It is a "branch" that has focused on central management (server) and the ability to stream videos. Many of the XBMC addons should work with Plex but you should inquire in the Plex forums before you count on them working.  Due to the differences though you'll need to use the Plex vesion of XBMC for your front ends, at least I think so.

I can't say about what's available in Canada (I'm in the US).

Another alternative is JRiver Media Center.  It is a pay application, but you get a lot for your $.  They are Windows based which does end up costing a little more.  They support a central server where all clients can stream from.  They support a ton of devices, netflix, etc.  About the only thing they don't support is cable card.  Their support forum is located at [Link: yabb.jriver.com]; If/when you have questions you can post in the forum and you'll get quick answers, often from the developers themselves.  I've posted about a bug and had a fix within 48 hours. (yes, 48 hours!).    Or you can open a support case in which you get direct support.  This is the best supported HTPC software there is.  On the down side it can be intimidating to set up.  It started as a music player and evolved into a full blown media player.  This evolution has left it being extremely flexible/feature laden which in turn makes it complicated to set up.