I second the PS Audio DL-III and the V-Dac. However, if you will only be connecting the iMac to the DAC, I recommend that you strongly consider USB only DACs, such as the HRT Music Streamer II+ ($350) and the Centrance DACport LX ($300) which will give you even more bang for your buck in your particular application. The computer digital interface is tricky, as there has been a lot of development in USB technology in the last couple of years, and these two USB DACs represent the results of two different approaches.
Keep in mind that many folks here (including myself) will rave about the value of the PS Audio DL-III and the Musical Fidelity V-DAC. But the praise most often refers to the performance of these DACs using coax or optical inputs, not the USB. If you plan on using USB connection, I would give the HRT II+ a strong consideration. There is also a new V-DAC-II out, which I believe now has an asynchronous USB interface, and thus might also be worth your consideration.
Keep in mind that many folks here (including myself) will rave about the value of the PS Audio DL-III and the Musical Fidelity V-DAC. But the praise most often refers to the performance of these DACs using coax or optical inputs, not the USB. If you plan on using USB connection, I would give the HRT II+ a strong consideration. There is also a new V-DAC-II out, which I believe now has an asynchronous USB interface, and thus might also be worth your consideration.