You can use any video card you want as long as your power supply meets its requirements.
With your 475w power supply, you should be able to use a GPU such as the HD6870 easily. Here is an example:http://www.newegg.com/Product/Product.aspx?Item=N82E16814102909
Note, that the GPU requires a 500w power supply, but the power supplies used in the HP workstations are actually pretty good and should work just fince withis this class of video card.
The bottom line is that just about any PCI-E video card will work in your system as long as it isn't too long. You can also add an aftermarket PSU without any trouble if you want to step up to a higher class of video card as well. You do not have to use an HP power supply in the Z400's case.