Your question has many answers in this forum... it is not a simple question or simple answer. With respect for the person trying to help you.... you need better more recent information:
First, is yours a 4-RAM-socket earlier version motherboard or a 6 socket one? Those have two different boot block dates, but if you have the 6 sockets you have the better later version.
Second, most Z400s came with a hotter running processor, 130W Max TDP, so look up your processor and if it is an original and has its original heatsink/fan it is already set up to run the hotter faster processors.
Assuming that is the case, then you may be exactly like I was..... had the better motherboard, and already had a "Performance heatsink/fan" that could run the latest fastest processors HP ever certified for that workstation. The later QuickSpecs show the latest processors... for example, version 56, HERE, page 3.
http://www.usedcomp.de/pdf/hp-z400-manual-handbuch-quickspecs.pdf
Note on page 3 the two bottom processors are both hexacores, run the fastest, lets fast RAM run the fastest, and yet there is one step up (which I am running). It turns out that for some reason these later Z400s can run an X5690 too.
So, depending on your motherboard, you might be interested in a W5690 or an X5690..... I chose X5690 for the one I'm typing on.... rock solid.
You may wish to read up on the difference between gaming cards versus enterprise cards..... we need to use Quadro cards because they have special drivers our medical software needs. I'd go for one step down in that nVidia line you noted.... not older, however. I like EVGA for those.