Changing double DIN stereo?

I want to change the stereo in my 2004 A4, the current one fitted is a double DIN cassette and in-dash 6 CD changer.
What can I fit in the double DIN gap? If I buy a standard single DIN radio
and facia plate adapter will this be made to fit the upper or lower part of the gap left?
I thought about a Blaupunkt Woodstock DAB54 (when they are released) and then something else underneath it, perhaps an in-dash CD or Minidisc changer, I just have no idea what I can fit there!
Thanks in advance
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