Here's the deal. Almost every sound card out there is quieter than the output of your radio with the volume at 0. And when you start turning up the volume, the sound card noise floor becomes less and less important because your RF noise floor coming through your audio chain is so much greater in comparison.
Let's say your audio card has a crappy noise floor of -80. Once you turn on your receiver and crank up the volume until you are getting a decent signal to decode, the RF noise floor is going to be around -50. (or -40)
Yes, the 30 dB lower noise floor adds in, but at a rate of 1/1000 of the audio from the RF noise floor. That ain't much even for a crappy audio card.
I have literally tried dozens of combinations of cheap to expensive sound cards and unless you are chasing signals WAY under your RF noise floor there just isn't enough difference to make a hill of beans. I did find some really cheap sound cards that had so much distortion that they refused to decode almost everything, but that had nothing to do with the noise floor.
Personally, I settled on the SignaLink USB because of it's price, size and convenience. I put toroids on the USB and the audio signal line, I did the mods listed below and I just use it. It's noise floor is WAY under where I work with any signal I'm trying to decode. In fact, the software decoders work better when you crank up the gain to just below where the decoder overloads. At that point, the sound card noise floor is about 70 dB down and absolutely irrelevant.
SignaLink USB Mods: http://www.frenning.dk/OZ1PIF_HOMEPAGE/SignaLinkUSB-mods.html
I believe if you did the AB8JV mod and swapped out the 600:600 transformers you'd be as good as doing all the mods. I don't know because the AB8JV mod showed up recently, but on top of all the other mods it managed to make a tremendous difference for the noise floor.
So, a $99 device, an hour of modding and $20 in parts puts it up to about as good as you can get at any price as far as decoding even weak signal modes.
If I wasn't full time employed, I'd make a cottage industry of modding SignaLink USB interfaces for other hams.