On most 24 bit cards the last 4 to 8 bits is dominated by noise, so although you may have 24 bit A/D converter in your card if fed by a precise voltage at some low level of voltage the readings start wandering all over the place and the result are meaningless.

So what you are really interested in is what is the real accuracy of the card? On a good card you may get as high as 20 bits of accuracy, on a modest card you might get 18 bits and on a so-so card you might get 16 bits of accuracy. Yet 16 bits of accuracy is better than a 16 bit card that only get 14 bits of accuracy. 16 bit cards suffer from the same problem but not as great an extent. So the bottom line is that a medium quality 24 bit card is way better that a great 16 bit card.

Always shoot for 24 bit cards when you can afford it, even if on a modest budget there are inexpensive 24 bit cards that are fairly good such as the Sound Blaster 24 PCI.

What has this to do with a receiver? When your signal is way down in level you want to make sure that signal is not in that bottom area where the readings are worthless, a poor card will give you a muddy sounding signal while a decent card with the same signal will be clear and readable. Which one do you want?