According to HP:
https://support.hp.com/us-en/product/hp-z210-convertible-minitower-workstation/5053199/model/5053204/document/c02794662#AbT1
Max ram is 16GB
you have 4 slots, and they can accept 1,2 or 4GB ram ddr3-1333 NON ECC or ECC but not a Mix of CC and NON ECC on the board.
so your looking for to max it then 4x4GB DDR3-1333 NON ECC is more common to find.
Note: you could attempt putting DDR3-1600 or 1866 in it, and in theory the ram will run at 1333, BUT I cannot guarantee it will actually work on your motherboard.
As for CPU upgrade, the best I make out for your motherboard is the following information :
http://www.cpu-upgrade.com/mb-Intel_(chipsets)/C206.html
For what CPU could be run on it. Again the best in that category would be E3-1290 v2, and pretty common to find them on Ebay, though 1285 v2 will be abundant and cheaper. (of course this all depends where you live on this planet)
you know your processor isn't bad at all, it is a 4c/8t system and if you compare it to an I7-3770 it is only 200 points lower than it on passmark web site , and if you upgraded to a E3-1290 v3, you would find performance rated cvomparable and possibly a little faster than an I7-3770K, which isn't much to be honest for the price tag attached to that CPU (ebay roll about 250-300$)
=1190&cmp[]=2&cmp[]=1774]https://www.cpubenchmark.net/compare.php?cmp[]=1190&cmp[]=2&cmp[]=1774
personally I would keep the CPU and get for the money a better video card, and since you HP comes with a 400 W power supply you could easily (if it fits in
the case (see note below)a 1060 6gb and would make very good gaming box.
Note:
regarding dimanesions of the case, you will have to check yourself but the HP web site states:
Standard minitower orientation: 447 x 178 x 455 mm (17.6 x 7 x 17.9 in)
so I think you would be good. the only hiccup I can see with your system does the power supply have 2x6+2 PCI power cords, if not then you would have to upgrade the power supply as well.
hope I answer your questions