mm a maxim DAC? I though about a pcm or I2S dac, maybe better for audio, with his own memory and clock system, to keep a good sync (hard to do with one pic). Making a hardware ring buffer shouldn't be hard. When the half of the buffer has been outputed, it make an interupt, handle by the pic which fill it with the new samples. So we have a lot of "time" to handle a user interface with the pic and why not some mixing? For example with the 18f452 (10Mips), we can make about 200 instruction for each data @ 48Khz. yeah, seems doable :) designing it shouldn't be a real problem too.