The price of a 500GB drive right now...http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010150014%20103530113&bop=And&Order=PRICE
If a 500GB drive goes for $60, it's foolish to even consider buying a smaller one...and, once your interests involve video, hard drive space disappears very rapidly. And you will eventually move in that direction
That's where I would start.
And I would get another...for backups. I don't believe in backing up to the same disk my O/S is on, I prefer to slightly lessen my chances for a personal disaster if one drive goes out.