Q: The MacMini (late2009) is it compatible w/Sierra?

It is hard to tell for sure, see > Get ready for macOS Sierra - Official Apple Support

If so, you will probable want at least 4GB of RAM like the previous OS Xs.

https://eshop.macsales.com/shop/mac-mini/

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This is what Apple said in their presentation

According to Apple, the official compatible hardware list of Macs capable of running Mac OS Sierra 10.12 is as follows:

• MacBook Pro (2010 and later)

• MacBook Air (2010 and later)

• Mac Mini (2010 and later)

• Mac Pro (2010 and later)

• MacBook (Late 2009 and later)

• iMac (Late 2009 and later)

I believe a Mac mini 2009 model will be a Macmini3,1 model and therefore is not supported by Apple to run macOS Sierra. It can run El Capitan.

Another way of looking at it is if it is an old shape Mac mini it is not supported, if it is a new shape it is.

This is an old shape Mac mini.

http://www.everymac.com/ultimate-mac-lookup/?search_keywords=Macmini3,1

This is a new(er) shape Mac mini.

http://www.everymac.com/ultimate-mac-lookup/?search_keywords=Macmini4,1