It is not recommended to load Hyper V on SBS 2011. SBS 2011 is already doing a lot with SharePoint, Exchange, DHCP, DNS, AD, SQL. It makes a good guest but a bad host.

You can host Windows 7 on Hyper V.

Windows 7 also allows 64 bit guests but not using Microsoft's Virtual PC. You will need to use something like VMWare Player (Free) or Workstation. You will need to enable a setting in the BIOS called Virtualization technology or something similar.

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I've heard Windows 8 can host 64-bit guests. How about I install Windows 8 64-bit in a box with 32GB of RAM, install Hyper V on it, then host a few Windows 7 Pro 64-bit machines?

I'm not sure a single Phenom can handle that smoothly. You might want to look into a FX-8150 or i5-3450/i7-3770 if you will be running 3 or 4 machines.