If I run "virtual WindowsXP" via Parallels on MacPro with "Nvidia Quadro 4000 video card made for Mac", -
would virtual Windows fully utilize or even be able to utilize that video card?
I use Fusion, not Parallels, and as The hatter says, you're best bet is to raise the question with Parallels support, but generally, the virtualizing software presents its own virtual graphics card to the VM and it's the virtualizing software which then makes better or worse use of your actual GPU behind the scenes. I assume when you say "virtual WindowsXP" you're referring to a virtual machine of XP rather than XPMode, which is a free "virtual WindowsXP" running within Windows 7 Pro as a "nested" virtual machine using VirtualPC.
I use to hear that "3D graphics improvements" in VM but it is a _VM_ and it is XP and afterall what drivers are used.
And why run in a VM if you want performance? And why XP? not like it uses your 16GB even though the VMs have upped how much they can shift and share. It isn't going to be like running natively in x64 Windows 7.