Finally found a web link for the Z series high power video card info I referenced:
http://h10032.www1.hp.com/ctg/Manual/c03276836
12V x 18A = 216 watts.... well beyond the consumer grade ATX standard of 75W per cable. Not that I'm recommending you push close to that limit. However, this is why HP is able to provide power adapters for the Z series supplemental PCIe video power cables. One of those adapters takes a 6-port plug/cable and converts it into a single 8-port plug, and the other takes a 6-port plug/cable and converts it into two 6-port plugs. So, for a 165W card there will be a lot of breathing room with even just one cable used.
Added good news is that video cards keep getting more powerful despite less power used.
Here's a screen shot of those HP adapters:
Image may be NSFW.
Clik here to view.
