Not a great question...
Depends what kind of person/gamer you are...
If you just play a few games.. but perhaps hard core.. and you organised.. 500GB will be WAY more than enough..
If you are the kind of person who wants to play every game under the sun and download every free game or bit of download content/mod for each game and are so disorganised you cant resist clicking on any download for anything.. you will soon fill up 500GB
So its up to you.. at least of you start with 500GB.. if you turn out you need more.. you can easily buy more.
So its not a critical decision that you must get right, right now....
Yeah figured that it wasn't the greatest of questions but im new to gaming pcs and i dont plan to be downloading games left and right but i do plan to get games like skyrim, battlefield 3, and so on. well thanks for the input