How are you going to get the video off the video camera on to your PC?
Normally you need a video capture card in your PC to do this, so you have something to plug your video camera in to.
Giving an amount of space that video takes is difficult as video can be copied at different quality formats. Same with an mp3 song, if it is low quality the file will be small, if top quality the file will be large.
But as a general comment video files can be very very large and you will not get 15 hours of video on a memory stick.
It may be a good idea to buy an external hard drive JUST to put the video on, and maybe also copy it all to DVD (2 copies of each) just to make sure.
If you are going to all the effort to put the stuff on to computer better to make 2 or 3 copies of it, rather than the one which may get lost or damaged.