The desired number is 24 bits, a 24 bit card has theoretically 16 times the resolution, and the dynamic range compared to a 16 bit card, and therefore you will have a better receiver, but watch out no one really makes a true 24 bit card, they lie through their teeth on this one. If we truly had a 24 bit card we would have an earth shaking receiver, but we are not there yet.

That does not mean that it doesn't make a difference because it does, different manufactures have varying amounts on how close they get to 24 bits, some are only 16 bits of usable resolution some are as high as 20 bits, you want the 20 bit card.

Here is a list of really good cards that perform rather well, as expected they are rather expensive;

Delta-44 and Delta-66 it cost about $125 excellent PCI card for your home computer. It has 4 inputs and 4 outputs hence the "44" as part of it's name, with this card you could run two receivers at the same time.

Edirol FA66 it cost about $250 it has even better specs than the Delta, it's an external sound card that uses Firewire to connect to your PC so it can be used with laptops also.

Lynx professional it cost about $1000, it's about the best commercial sound card available now, it's a external sound card with Firewire connection.

HPSDR.org group Atlas/Ozy/Janus its about $350 for the motherboard and the two cards, it's quality is about the theoretical limit using present technology, it's a set of boards with no case so you are on your own but the performance is breathtaking.

There are several other high priced but with decent performance but I don't have experience or know anyone that does. With the rest the price varies quite a bit but they vary in quality all over the place. More money in this case does not mean better performance, some of the expensive consumer cards can be dogs for SDR use why? Many have excessive noise beyond the human hearing, good enough for audio but not so good for SDR work.There are some good performing cards for low cost to be had, as an example;

Sound Blaster Live 24

Unfortunately this card is not manufactured any more but you can buy them on eBay for about $10 to $20, the card performs quite well getting close to the performance of a Delta-44 but at a minuscule price. The internal Aux input is even quieter than the outside connector. There are some peculiarities with this card, the two samples do not occur at the same time, very common with Sound Blaster cards and other inexpensive cards. Fortunately the software for SDR use for the most part compensate for this oddness. A indication that your software is not compensating or you have not turned on the feature is that you get images of signals on both sides of the band-pass.

Some of it's higher price siblings do not fare so well a very popular card the SB Audigy 2 SZ has some issues with noise specially on the output side, the noise is outside of the audio range so most don't care or even notice it, but our transmitters will send that noise all over the place.