Hello FotoKaleART,
QuickSpecs for the Z820 show exactly what you already mentioned. The Nvidia and AMD Firepro cards are the only tested and qualified cards with drivers. However, that doesn't necessarily mean the GTX cards will not work. It does mean there are no specific drivers developed for this Z820 and those cards, and I cannot guarantee success with the GTX cards.
To view the qualified cards see the Quickspecs:
http://www8.hp.com/h20195/v2/GetPDF.aspx/c04111177.pdf
Hope this helps.
Sincerely,
Wendy_R
