If you have $500 you can get the Protoold M-Box, it is a usb state of the are capture device. Or you can get in eBay an Audio Media 3 card (PCI), it is the same thing like the M-Box but PCI.
Now, the problem is not the sample/ bit rate. Almost all imput/output audio device goes by those standarts. The problme is the harmonic distortion and the sample algorythm (1 bit ADC, Dual Oversampling, etc).
Now, there is not too much you can do about the algorythm, but about the harmonic distortion you can do something about it. When you read the instruction you will see something like "01% hamonic distortion", that means how accurate is the captured audio from the original signal, even if is at 44.1 at 96bit.
The mac audio i/o are very dirty, if you ever have the chance play CD from your Mac trhu your stereo and then play the same song from the stereo CD player.
I did that with a MP3 from my Powerbook and with the Audio Media 3 from my tower and the difference is from norml to DVD quality! You can listen to the base, not just the base sound if not the ambient generated by that particular sound.
The $30 Griffin iMic, won't be any good, just regular stuff. The $250 M-Audio Duo - 24-bit/96kHz converters (I haven't try them) and excellent sound quality, may bea better choice. Be sure these options are compatible with your audio software. Protools dosn't accept 3rd party producs for example.
BTW, there are not professional audio hardware/software for OSX.