This does get confusing as borders change and time goes by. However, I do believe you would still apply for German citizenship versus Polish. Ho are you related to your Prussian relatives? Anyone else have experience or can clarify?

In general (via http://germany.usembassy.gov/acs/dual_nationality/),

I would also recommend consulting the German Federal Foreign Office: http://www.auswaertiges-amt.de/EN/Infoservice/FAQ/Staatsangehoerigkeit/Uebersicht.html?nn=479790

The real place to start is with the closest embassy. Contact them and they can help guide you through the paperwork and process. You can also consult an immigration layer, but obviously this is a more expensive option.

For more options in applying for citizenship, visas, etc. - check out out Expat Guides, like the Guide to Berlin. Please - if anyone else has experience - share!