The border between DSP and microcontroller has blurred considerably in recent years. Many microcontrollers now have DSP functions (single cycle multiply accumulate), and some DSP's now have RISC instructions sets and microcontroller-style architectures. Also, it's common to program DSP and microcontroller functions into gate arrays such as Xilinx. Frankly, if I was starting today to learn about DSP's, I'd seriously look at becoming familiar with FPGA's, since that gives you a lot of flexibility with processing and interfaces. Maybe look first at the Xilinx Spartan 3e FPGA as a useful point of reference.