Curious. If you have the knowledge to check the size of your songs and movies folders, how come you don't know how to check how much memory space is left on your laptop? Not a criticism, but its easier to ascertain that, then it is the former.
Just click 'my computer', and then move your mouse cursor over 'Boot (C) - your hard drive, it will tell you how much space you've used, and how much remains. Alternatively, right click and check properties.
If you opt for an external hard drive, simply transfer the songs over, and when using itunes add this drive to your library. As long as its connected you can plug your ipod into your laptop and transfer music to it.