i followed the fantastic guide by powerhouse >> http://forums.linuxmint.com/viewtopic.php?t=112013&f=42 to setup vga passthrough, and it worked flawlessly !
i am only having 1 real issues:
amd radeon 6570 dom0 no hdmi sound. i tried installing the fglrx driver, but black screen in xen,
and fglrx working in non xen boot.
i edited grub to include radeon.audio=1 , updated it, and still no sound.
ati 6570 dom0
ati 7850 domU
like i said, xen boot works great for domU -- no problems there. just the sound on dom0.
if you need more info, i will gladly supply it.