One detail: the Family Pack was an upgrade license. It's not intended to be installed on a system with no OS.
Also: Windows 7 Home Premium X64 supports 16GB of RAM. If you want more (up to 192GB, if memory serves), you'd need Pro or Ultimate.
The least expensive license that I know of for a new PC is the System Builder (generic OEM) one, but the license is intended for systems for resale. (As far as I know, there is no enforcement of that by Microsoft.)