T420s and VT-d video passthrough?

[stumilowy]

Hello,

I want to buy an i5-powered T420s. It will be running Linux, and a few virtualized guest operating system (among other Windows 7 and Windows XP), probably through Xen.
I would be delighted to use new VT-d features - especially for direct graphics access from virtual machine - in hope to experience better 3d in guest OS. Do I assume correctly that I would benefit there from the second graphics chip (nvidia 4200m)?
I probably will not use at the same time two 3d-greedy apps on different OS-es - just one.
Could someone explain me if the VT-d with second chip is worth it, or maybe I will not notice real difference between VT-d and just standard 3d virtualization?

The thing is I could not find an i5 T420s with nvidia (BTW - could somebody explain why is there an i3 T420si with nvidia and without VT-d, but i5 is only with intel?). And I would prefer not to pay extra for the i7 option.

I would appreciate if somebody shares any VT-d experiences.