Welcome to Interact!
It seems like driver support is the key factor which is turning out to be pretty hard to research. Does anyone have any opinions about ASIO drivers for the above cards (or any cards that suit my purpose)?
ASIO was not designed with multiple instances in mind. Media Center supports multiple instances with a workaround, but most ASIO drivers do not allow opening them multiple times at once. This might be an argument for multiple cards. Or it might force you to use DirectSound or WASAPI. But then you'll bump into the issue where not too many cards allow presenting their 8 channels as 4 discrete stereo sets.
I realize I may need multiple cards to drive all channels. How well does multi-zone work in MC with multiple sound cards? Is a multi-card solution generally better or worse than a single card? Pros/Cons?
Zones work fine with any number of cards.
Zone sync (i.e. playing the same thing in all zones at the same time) may get a little tougher as you add more zones or hardware.
One feature that I'd like to take advantage of is MC's ability to output a stereo mp3 as mono. What I want to do is output one mp3 as mono to the right channel of an amplifier zone, and another mono mp3 to the left channel. This way I can use the amp's 12 channels discretely. The R/L speakers will be in separate rooms of course so you aren't hearing two different songs at the same time.
You can output mono with Media Center, but again you'll have to wrestle with the ASIO instance limit.
Is it possible to do this at all in MC? Am I going to run into processor issues trying to play 12 mp3s at the same time? It doesn't seem like a very taxing activity at this point in time. Does mp3 decoding get offloaded to the soundcard or does it need cpu time?
This is the sort of thing where a multi-core processor will really help. I'd consider a four core processor for a server doing this much work, but you would probably be fine with a fast dual-core setup assuming you're not doing heavy DSP on the 12 outputs.
Good luck, and let us know how you fare.