I use an M-Audio Audiophile Firewire with my MAC for two way data stream. The USB model can be found used on Ebay for around $100. I just compared the Monarchy NM24 DAC to the M-Audio, and while the Monarchy had more hyper-detail, I enjoyed music through the M-Audio device better.
For the money, I think you would be very happy.
For the money, I think you would be very happy.