Q:

What is good about Intel Vt-d technology for direct IO? 

I'm interested in 2 CPUs ( or I should APUs ? )  the main difference between this 2 units it's the presence of a Vt-d feature that Intel names "Intel® Virtualization Technology for Directed I/O (VT-d)". 
Since I'm also interested in running virtualized environment on this machine, what kind of features I will get or miss in real world scenarios ?  What this Vt-d is about ? 

A:

From this excellent Intel blog post which is at the top of the google results for vt-d:

VT-d, at the time of this writing, includes four key capabilities

I/O device assignment.  This feature allows an administrator to assign I/O devices to VMs in any desired configuration. 
DMA remapping.  Supports address translations for device DMA data transfers. 
Interrupt remapping.  Provides VM routing and isolation of device interrupts. 
Reliability features.  Reports and records system software DMA and interrupt erros that may otherwise corrupt memory of impact VM
  isolation. 
Note that VT-d is not dependent on VT-x. That is, a VT-x enabled
  system can operate without VT-d, or without VT-d enabled or
  configured.  You simply miss the benefits of the feature.  Many people
  have asked about this point. 
You can read about VT-d in detail at:
http://www.intel.com/technology/virtualization/technology.htm?iid=tech_vt+tech

This link is the architectural spec for VT-d, if you have an afternoon to burn and really want to know how it works.