Well, I finally talked to someone at fiire.
They say the fiire engine (the server, what pluto calls the core, mythtv the backend) is a M2NPV-VMhttp://www.asus.com/products4.aspx?l1=3&l2=101&l3=0&model=1138&modelmenu=1
has 2 pci slots.
My query about what to do with the other cards was that i could put 2 PVR cards into the fiire station (thin-client, what pluto calls the media director, mythtv the frontend).
I was sort of surprised as I expected everything to be in the server, although it balances the load...still :-
1) I wouldn't have thought a thin-client could handle Two PVR's encoding signals whilst receiving a movie from the server and outputting it to a TV, all of which, could be HDTV and, of course, sending those Two PVR data streams back to the server simultaneously for storage.
2) As that is, effectively, dual backend/frontend, I wasn't even sure linuxmce/pluto could do that although mythTV can.
I'll leave it to people with more experience and knowledge to comment on this. I'm just reporting what I was told.