To answer your question about Virtual PC or Windows 2008 R2 and VT-d, neither currently take advantage of VT-d (Direct I/O assignement). With the release of RemoteFX in SP1 for Windows 2008 R2, VT-d will be supported and you'll be able to have Hyper-V take advantage of a GPU in the system for DX9 and DX10 workloads. For Virtual PC, I'm unsure if or when it will support VT-d. VT-x (processor virtualization) is supported by both products.
It sounds to me like you are looking for a single platform where you can run multiple operating systems at the same time without having to worry about reimaging your host OS each time you need to change OSes. I've done this type of setup before and I think you will find that a single desktop OS with a type 2 hypervisor software will meet your needs better than running a Hyper-V server. Hyper-V is more built around the idea of multiple users connecting into the Hyper-V server on remote terminals rather than working directly on the server. As an alternative, there are other type 2 hypervisor solutions available that do take advantage of VT-d. Virtual Box (free), VMWare Workstation, and Parallels Workstation Extreme all support VT-d and are built around the usages of a single user running multiple VMs at the same time.
If I've missed the core of what you are trying to do, then feel free to respond with additional details around what you are looking to accomplish.