Hello,
Yes, you can do this. Wowza automatically supports streaming to different devices at the same time. If you are new to Wowza a good place to start is with the VOD Tutorial. Look in the “Playback” section for examples of how to play to different devices at the same time.
As far as playing live from your capture card, you can do this if your capture card can provide a live stream to Wowza in one of the formats Wowza supports, then the stream can be viewed live. If your capture card can record to a quicktime container, it might be possible to stream the file before it is finished writing, but I am not familiar with this method.
Regarding server specifications. The limiting factor is usually bandwidth. Make sure your server has enough bandwidth for your desired number of clients and bitrate. On the hardware level, basic dual core 2Ghz servers with 4GB of RAM can stream 1500-2000 streams.