I heartily agree, it's incredibly difficult to find any useful information on the web concerning sound cards, at least compared to the processor, video card, or motherboard.
Someone mentioned the GUS and that it rules, to which I can only say YES! I still have mine (in GUS MAX form), and would be using it today if it was supported in Win98. I paid $200 for it about 6 years ago, and at the time it was far better than any Soundblaster product, way ahead of it's time. But I digress..
People have mentioned lots of good things to test on soundcards. Of particular intrest to me would be the installation/compatibility. For instance, reading through the info provided by retailers (either local or online), you'd have no idea that any soundcard based on the Vortex2 chipset (ie MX300, many Turtle Beach cards, and others...) are NOT COMPATIBLE with Athlon chipsets. This according to Turtle Beach support. Nice of people to tell you that BEFORE you buy it! Of course, I've read a great deal of negative comments regarding use of the SB Live! on Athlon boards as well.
A feature list would be great too, as it can be rather difficult based solely on a brief product description, to tell whether or not a sound card has the specific input/output that you are looking for. Some online vendors are better at detail than others, but even the best could use a little improvement.
Cheers,
Aaron