I've used microSD card and not managed much faster than 3--4ms per block (512 bytes) through the hardware SPI interface at 4MHz clock, which is about 140kB/s, fast enough to read enough samples to drive a DAC, but the DAC would not be able to share hardware SPI - perhaps 8 bit parallel by directly writing to the AVR ports...
But if you want to write as well that's slower, perhaps 6ms to write, 3ms to read. overall throughput 56kB/s, not really enough - and that's not including any overhead for doing any interfacing to ADC/DAC or other Arduinos
So I'd say it would be very difficult without more powerful hardware, specifically something with enough fast RAM on the system bus to do the buffering (1MB ought to do it), then you only have to write the interrupt handlers and interface suitable DAC and ADC. A DSP based board is likely to be far better bet... Anyone know an open DSP board?