The operating system on the render pc is not critical. I have mixed various flavors of Win 7 and even XP without iussues. If you have a license available I would definitely go with a 64bit version. For windows 7 it is useful to have pro or ultimate as they can act as servers for remote desktop, meaning you can control them form your main pc via remote. My render PCs don't have monitors, so that way I can easily control them.

Ciao

TD