My Games HD, where I also keep some FRAPS recordings and a few backups of valuable files, is ~500 GB. I just yesterday had to uninstall a bunch of games that I didn't have a particular likelihood of playing anytime soon in order to clear up space.
If you play a lot of games, I would recommend at least 500 GB to avoid frequent downloading and uninstalling. 600~800 GB would be a nice bit of room to be comfortable with.
At least 60 GB for your OS, and if it's a SSD, keep your page file somewhere else.
Then I would recommend a separate HD or partition for your various programs/applications and files (like music and pictures), space varies depending on your needs, but ~50 GB should cover it pretty well, unless you keep a bunch of HD movies and stuff around. Or you could just keep that on the big HD, but it gets very cluttered so I like to keep it separate