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[Xen-users] AMD-Vi/Intel VT-d: Passthrough or virtualization?

For background about why I'm asking this, and some other things, you may want to optionally read this Thread:
Also, keep in mind that I have no Xen experience - all my virtualization has been done using Microsoft Virtual PC 6, so my practical knowledge is limited. But I want to run Xen, after I get the proper Hardware to run it like I want to.

When I started to hear about virtualization around 5 or 6 years ago, one of the main issues at that time was that for the average Windows user it was mostly useless for just one reason: You couldn't actually use the Video Card. And that means no gaming on a virtualized enviroment.
Some time later, I started to hear about Xen supporting "VGA passthrough", that allowed a Virtual Machine to see and use the Video Card. If I recall correctly, it was in these videos:

Because the machine that as been used for these videos is supposed to be a Core 2 Duo E6300, it couldn't have any type of IOMMU virtualization as that feature, Intel VT-d, was introduced one generation later, starting with Nehalem based Processors. However, everytime I google around I hear people saying that in order to do VGA passthrough, you need AMD-Vi or Intel VT-d, when according to those old videos passthrough was a different thing that didn't requiere IOMMU virtualization support for it and that is why it worked for him.

So, to resume this, can anyone explain or give a documentation that can exactly tell what is the difference, pros or cons, bewthem that old passthrough that I saw and what you can currently do with AMD-Vi/Intel VT-d? At least, if I recall correctly, the problem with that type of passthrough is that only ONE Virtual Machine can make use of the Video Card. If I virtualize it, can multiple VMs share the same GPU, like the OSes currently do with the CPU? What are the limitations?
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