The minimum amount of space you'd want for general use (programs, temporary files, paging file, etc) with xp or vista is like 100 gigs. Then figure out how much space you'd actually use and add that to the amount . Now, you want to make sure you leave at least 25% of your boot drive unused for performance reasons (and future usage), so divide what you get by 0.75 and you'll have the minimum amount of space you need. Then try and guess how much extra you want (the more extra space you get the more time you get before having to upgrade), and add that all up, and you end up with the amount of storage to get.
Like with me, I have about 700 gigs of movies/music/downloads and about 500 gigs for my programs (I have tons of games), and I know I will always need about 200 gigs as a temporary area to store downloads before I move them over to storage. Right now i have 1TB on my main volume and 750gigs (FULL
) for my storage. But as soon as the velociraptors come out I'll be at about 1TB for the main volume and 1.5TB for storage.
So yeah, its really up to you.