If you want only 2 channels (with preamps included) the Apogee Duet is a perfect choice for the Mac.
http://www.apogeedigital.com/products/duet.php
Around 500 dollars USD.
RME, MOTU, PreSonus all have versions as well but some lack the support Apogee has for Macs.
The second option would be a metric halo unit (bit costly, but amazing).
http://www.mhlabs.com/metric_halo/ . The ULN-2 is a fine piece of machine
http://www.mhsecure.com/v5mm/ULN2_2dExpanded.html but that is a nice bit more than the duet.
The last route would be to get a high end convector (ie Lynx Aurora, Apogee Rosetta, Lavry, Burl, Prism etc) and choose your preamp (API, Neve, Grace, PreSonus, Focusrite, ART, etc). Which is the ultimate in customization, but the most expensive route listed.