Brad has an Apogee duet professional sound card. He'd like to upgrade to a Duet 2, but there's other less expensive options out there. Leo says that professional audio is a very specialized niche market, which makes it hard to offer mainsteam options. He also says that while newer Macs have more speed, Firewire is dead now. Quality audio processing is key, and he recommends looking at a DAC from ApogeeDigital. Low latency is also important.
Dr. F8, in the chatroom says that the RME drivers are better than Apogee, for what it's worth.