And to what extent does this card have support in Linux?
It exists, though the newer the better as far as this is concerned. The methods of getting it working in Ubuntu 8 & 9 were kludgey, but worked. Strictly speaking, there is driver support. Whether or not the sound applications can use it may be another story.
Mine worked (tested on ubuntu 10.04 & 10.10 ). But to get multichannel it needs to be connected when the computer is being powered on. I have issues also - mine would shut off during 30-90 min. of playback. During driver initialization it sends a spike to the amplifier. Looks like the firmware/hardware/software is not doing well to prevent it. Don`t know yet why.