You don't state what type of card or what its total power draw will be. I've never had a problem with the HP-provided PCIe supplemental power cables providing what I needed for any video card wired as they are, but your needs might be different.
Here is a great resource, directly from nVidia:
http://www.nvidia.com/content/quadro/pdf/quadro-power-guidelines.pdf
The PCIe x16 video slots have been rated for up to 75W from the slot itself, and the supplemental cables generally provide an added 75W.... this info will be in the technical and service manual for your Z620, which you can find via google and download. So, up to 150W is easy. You also can aggregate other power sources to get to the 8-port standard with quality adapters. The xw6400 provided less, and the xw6600 came up in power. The Z600 and Z620 have been up in power supply through their video slots too.
I believe HP's engineering would be providing you PCIe supplemental video power cables with above, rather than below, the ATX standards.