Hi guys, just a warning here, you can seriously damage your HP z800 power supply if you use the gaming cards.
Trust me I overloaded the rail with a GTX 560TI card and needed to get a new power supply unit. These workstations are not designed to run the Gaming cards, but cope well with the workstation cards. And they are pricey true, but way over the gaming cards. So if you are looking for a CAD machine then dont waste your time with the Gaming cards they are too power hungry and will damage your machine.
Here is the HP recommended list
http://h20195.www2.hp.com/V2/GetPDF.aspx/4AA4-6106ENW.pdf
Your current card has the following rating
NVIDIA Quadro 2000 => 1310
Else look at these
Quadro K2200 => 3523
Quadro M4000 => 6740
Quadro M5000 => 8508
Quadro M6000 => 10091
SLI Certified
2x Quadro M5000 => 16000
2x Quadro M6000 => 20000
The last one is a monster card, probably worth more than the PC itself
You can also setup 2x these cards if you have the 1100w power supply.
You could drive 8 monitors with two M6000 in SLI and they will work, with HP guarantee for 24x7x365, )k it will cost you around $4,000, but you did ask what can you do with a HP Z800.... quite a lot is seems