A PC laptop should have 750GB hard drive space. This space is needed for storing pictures, mail, music, games and other things you will be downloading on your PC laptop.
That will depend very much on how you use your computer. if you only do office work, web surfing, and listen to a little music, a 40 GB hard drive with Windows Vista, or even a 10 GB hard drive with...
you can use it in an external enclosure that connects to the laptop using either USB, firewire, etc.
However, you will never have enough room to install a genuine hard disk inside a decent laptop...