I'm running a Windows 7 64-bit host, with Microsoft Virtual PC 64-bit installed.
With this setup, can I install Windows 7 64-bit in a virtual machine (VHD file) as a client OS?
Microsoft Virtual PC requires you to select an architecture so it can install the correct version. However, neither support x64/64-bit guests.
I would advise you install VMWare Player which will allow you to install a x64/64-bit guest on pretty much any modern machine.