HP Workstation Z400 Graphics Card Update to FirePro Graphics V7900 - 6 Beep (red flash) Error

[DAVIDs10]

Hi all.  I did a lot of research before I attempted this.  I wanted to upgrade my Z400 Workstation to game better for Flight Sim 10 (FSX) frame rates.  My workstation came with a NVIDIA Quadro FX 580.  I wanted better frame rates in the sim.  I researched the system specs.  They showed for the Z400 Workstation that my high end 3D card compatible with my system is the AMD FirePro V7900 2GB Graphics.  So, I went in, deleted the NVIDIA drivers, shut down the computer and disconnected everything.  Removed the old card and installed the new one. The computer started screaming at me, 6 beeps and red flashes on the power button, the  pause and beeping, red light starting again.  My current system:

BIOS:  786G3 v03.20  11-10-09 (date)

MOB:  Product: 0BACH, Serial 2UA14708CT, Version D

Chipset Intel X58 Express

475W PSU

Intel Xeon W3570 3.20Ghz CPU according to specs. (which I installed with the larger heat sink and no issues, works fine)

Windows 7 Professional, 64

RAM 8GB

I looked into the BIOS to do Legacy Support = Enable, Secure Boot = Disable, this option does not exist in my BIOS.

It just shows a list of items to set up passwords...

Researching the error codes for the 6 beep/red flash situation:

"This indicates a pre-video graphic card error."

To resolve this issue, complete the follow steps:

1) Reset the graphics card (Done.  It fits fine in the slot as the original card)

2) Replace the graphics card (graphics card brand new out of the box)

3) Replace the system board (please don't have me go there!)

So, why won't the new $250 card work in a system when the specs indicate it's a compatible card?  Can I even do anyting in the BIOS considering the version listed above? Does this card require a larger power supply or some kind of connection to the existing power supply?

I really thought I did my homework on this one.  So if there are no options with my current system (except changing out the motherboard) What can I do to upgrade the video card for maximum gaming without going through some kind of monstrous rebuild?  Then of course returning the new card to get my money back!

Thanks in advance for any assistance out there.  Reviewing a lot of subjects and answers we have a tremendous amount of knowledge in here and I am very impressed!