tell application "Finder"
    # Finder gets the free space in bytes
    set free_space to the free space of the startup disk

    # convert to MB
    set free_space_in_MB to free_space / 1.0E+6 as integer

    # How many whole GB?
    set free_space_in_GB to round (free_space_in_MB / 1000) rounding down

    # How many MB remaining once we have subracted the GB?
    set remaining_MB to free_space_in_MB - free_space_in_GB * 1000

    # Et voilá!
    say "There is " & free_space_in_GB & " gigabytes and " & remaining_MB & " megabytes of free space left on the disk."
end tell