I just did a clean install of Windows 7 Home Premium on my laptop last night. After deleting some bloatware, disabling Hibernation, moved the Pagefile to the D: drive and install some components of MS Office 2007 along with some other minor programs Win 7 took up about 15GB. I really can't see Win 7 Pro taking up much more space than Win 7 HP... at most maybe another 1GB?
I would say 64GB is more than enough if you are only going to install 1 game. Games are typically 7GB - 15GB so you can install up to 3 games and still have plenty of space for the saved game files.