Hello-
So the cards we have tested are exactly what you metioned in your post:
http://www8.hp.com/h20195/v2/GetPDF.aspx/c04111177.pdf
FirePro, Nvidia both quadro and tesla. The GTX cards have not been qualified or tested, however, that certainly doesn't mean those cards will not work. Simply that there has been no formal testing or drivers created specifically by HP for the Z820 and GTX video cards.
Please let me know if this helps.
Sincerely,
Wendy_R