My new setup works nicefully with PCI Passthrough (very easy to setup, all standard package from Ubuntu 12.04, no patch).
here is my hw :
core i7-3770 (do not take the "K" because it doesn't have iommu)
asrock Z77 Pro4-M
sapphire radeon HD 7950
basically I have xen 4.1.2, a Dom0 running Ubuntu 12.04, on the Intel HD4000 graphique card. (connected in DVI to my screen).
and a HVM DomU (windows8 release preview) on the radeon HD 7950 (connected on the same screen with HDMI).
so I can switch from Linux/Windows by changing the source on the screen. (1 button).
I can share my settings if someone need it.
also in the future I may add an ubuntu PVM which will use the radeon HD7950 for Linux game.