I am not sure where your going with this, since most SDR 'interfaces' are the I/Q lines to a sound card....the sound card is important, and is usually PCI/PCI-X, USB, or Firewire.
I don't think he is asking about old outdated sound card based SDRs like the Softrocks, Genesis, LD-1, SDR-1000, etc... He is asking more about the modern direct sampling SDRs like the Perseus, QS1R, RF Space, or Winradio Excalibur. I would not say that "most SDRs" use I/Q to the soundcard either. The modern direct sampling SDRs provide bandwidths much greater than 192 kHz. The old fashioned sound card QSD based SDRs are limited in bandwidth by the common sound card rates of 48 kHz, 96 kHz, and 192 kHz.