You should not need cables at all. First, you will need to identify the type of xbox you have. If you have a "fat" (curved sides, no vent on side), you purchased the wrong hard drive. If your Xbox is "slim", (angled or straight sides, vent on the side) then you will need to remove the small vent on the side of the Xbox and slide the drive in.
Yes it will fit the hard drive slot is empty the 4 gigs that come with the system are internally on the motherboard so you can use both at the same time but they will show up as two different memory sources to choose from.