In summary:
64GB
Barely enough for the OS and a few other games etc. OS updates and SP1 etc. will fill this up fast. Don't do this unless budget demands it.
128GB
The present "sweet spot." Plenty of room for the OS, Windows Office, and several games.
256GB
Even better. You can put pretty much everything on this except for movies/music.
512GB
At this level you can consider foregoing your HDD completely. Gobs of room for everything unless you have an extensive pirated movie collection or use your computer in a professional environment. This is what I will use for my upcoming computer combined with cloud storage.