Windows 7 Home Premium is probably your best option. For a comparison of Windows 7 editions's features please see:
http://windows.microsoft.com/en-us/windows7/products/compare?T1=tab15 .
You should also consider whether you want to install the 32-Bit or 64-Bit version of Windows 7. If its going to be a high end gaming PC you're likely to want 64-Bit. Please see:
http://windows.microsoft.com/en-us/windows7/32-bit-and-64-bit-Windows-frequently-asked-questions
for further important information.
As its going to be for gaming you should know that Windows 7 64-Bit does not support 16-Bit code. this means that very old games may not install/open. The only solution to this is to use a virtual environment.