And by audio support, do you mean one that can pass through the spdif of your motherboard integrated audio onto the HDMI cable, or do you mean one that has full 7.1 PCM support? There is a big difference.
Not to nitpick, but I will anyway. PCI-X is a slot designed mainly for servers, it is a higher-bandwidth version of PCI.
PCI-Express is the fairly new consumer slot, known as PCI-E. http://en.wikipedia.org/wiki/PCI-X