For me, (max) 3 inputs and (max) 3 outputs would be perfect.
Yes, I'd be using a multi-channel amp to drive the speakers. Today all of the speakers are wired individually to the PC closet, where they are then bundled together onto one pair of outputs (a big amp, to handle that sort of impendence). I want to move each pair of speakers onto its own pair of outputs, but first I need the software to drive them that way.
I would consider a solution where each zone had its own PC, but would prefer it if one PC could drive all 3 zones. I could only accept a multi-PC solution if it enabled all of the PCs to play perfectly (and automatically) in sync with each other, and if the remote control capability still worked. I tend to think it'd be simpler to do it all on one PC (from your perspective).
Whether the output is analog or digital isn't really important, although it'd be nice if it supported both. My testing with M-Audio 1010 and Proceed AVP shows that the best sound cards these days have just as good sound quality as the best preamps.