VGA Passthrough Almost Working!

I almost have VGA Passthrough working in Xen.  The AMD Radeon card is showing up in the device list, but its not working yet.

PCI passthrough of a Sapphire Radeon HD6570 using KVM did not work.  During the Windows XP
install, the entire box (virtual and physical) froze.  After rebooting, the installer wouldn't
work anymore.  I recreated the virtual disk then tried the install over again; this time without
the PCI passthrough.  The install succeeded.  I shut down the vm, added back the PCI passthrough
and brought up the vm.  After the boot (around the time Windows detects new hardware), the entire
box froze again.  Its worth noting that I'm passing through a secondary video card.  I have a
GeForce GTS 250 that serves as the physical's video card.  My next step was to try using Xen.
Although I feel Xen is faster than KVM, KVM is much easier to configure and use.