The two things that are going to  affect your perception of how a sound card sounds the most are its frequency response and its SNR. 

 

If the frequency response is within +/- 0.2 dB from 80  to 13 KHz and does not peak or dip excessively (say more than +/- 3 dB)  32 Hz-17 KHz,  then the card is probably going not going to color the sound in such a way that it will bother you. 

 

If the SNR is above about 75-80 dB, then noise is not going to be too much of an issue as long as you keep your recording levels up. In practice much of the audible benefit of SNR's > 80 dB is that you don't have to be so careful about setting levels because if you are a little on the low side, the noise from the card still won't intrude. 

I love the better cards  in my collection (above the gold line) because their low noise makes setting levels very easy.. If the peaks only make it to -10 dB so what? I'll make  up the difference in the editing software after the actual raw recording (tracking)  is completed.

 

Since sound card SNR test results are generally tied to IM & THD test results by the mechanics of how sound cards work, if the SNR is above 80 dB, the THD and IM for the card are also probably going to be just fine.