Using a mic plugged into the sound cards means you're dependent on the A-to-D converters on the sound card. Those converters have to run in a truly hostile environment, in terms of electrical noise at a huge range of frequencies, there inside the computer case.

A good ordinary microphone plugged into an external USB/Firewire sound interface gets around that problem, as does a "USB microphone" since it has the A-to-D built in. Of course then the question of how good that A-to-D is becomes relevant.

For recording human speaking voice, you don't need wonderful equipment, and keeping actual background sounds down probably makes more difference than picking equipment carefully.