Here are a couple reference posts with all you might want to know about DSP amps.
First, is a buyer's guide that summarizes the known differences between the part numbers and major transitions.
DSP Amp Thoughts and Buyer's Advice
Second is a list of the known part numbers. These are all backwards compatible. You can probably use an older one in your car, but I would recommend an amp equal or newer to the one in your car, just to be safe.
DSP Amp Part Numbers
Finally, after you get your amp, you will want to get it coded by a dealer or BMW specialist. This will ensure that it matches your car's other options and audio characteristics.