Hey there, Mastiff!
1. Is there anything that can stop me from sending two 720p video streams out from the same card?
Assumuing that you are NOT including AACS-protected stuff (and/or other forms of DRM), then the only thing limiting you would really be memory, CPU, and decode filter performance. My Core i5 750 @ 4.0GHz machine can handle dual simultaneous H264 1080p renders (one on each monitor). I do
occasionally hit disk performance issues right when starting the files up at the same time as one another, though (light stuttering just in the first 1-3 seconds and/or a longer-than normal loading delay). Of course, many of my HD rips and recordings are pretty high bitrate (10-14mbps). I don't know if you'd get more than that, though. I've never tried.
Decoding all of that stuff is going to be pretty hard on a server's system, though. Saying for sure would be impossible without trying, but you have a very good shot at getting 4 files decoding at once.
2. Will the HDMI audio on the two ports of a graphic card be adressable as two zones, or do I have to go for using regular sound cards on one of them?
Not sure what cards specifically you are going to use, but a few things:
1. There will almost certainly only be one audio "device" per card, regardless of how many HDMI or DVI ports the cards offer. My Radeon HD 6870 only has one.
2. Older video cards have varying degrees of HDMI audio support, ranging from none at all, to limited support. If you are going to use these for anything other than media playback (sounds like no) then you could hit issues with multi-channel audio, unless the card is carefully selected. Many of them do not support full multichannel PCM bitstreaming, and some don't have Dolby "Live" Encoders, which means they can only pass AC3 or DTS through, otherwise they are effectively 2-channel sound cards. This may or may not be a problem depending on the "reciever" you use at the other end, and how you want it to handle things.
3. You'll basically need Nvidia's GTX 4x0+ or AMD Radeon HD 5x00 to support bitstreaming Dolby TrueHD or DTS-MA if you have a way to decode that. If you care at all, only the very latest generation (AMD 6x00 or Nvidia 5x0 series) support HDMI 1.4, for the funny looking glasses.