Proliant raid controller in a PC
Solution 1:
I haven't done it personally, but so long as you have the right slot, cables, drives the controller supports, and a driver for the OS you intend to run it should work fine. The ProLiant RAID controllers do not have any reliance on being installed in a ProLiant server computer.
The driver CD that comes with the controller probably won't work on a white-box PC but you should be able to use the ROM BIOS interface (HP calls it the "ORCA" interface). The P400 won't offer this ROM BIOS interface prompt unless it detects supported drives attached.
Solution 2:
It's do-able. I've run Smart Array controllers in non-HP server equipment (Dells, Supermicro, etc.). You'll have to use the ROM BIOS utility to get your initial configuration. Advanced config is possible with the relevant HP Array Configuration utility for your OS.
Solution 3:
P400 series are PCI-Express and should be accessible after boot POST within most PC.
In my case, I used a Lenovo and the P410 controller.
The things you need to do is to tweak 5.25 front arrays for drives bay and a little MOLEX adaptation following the wire map for your device.