It's a 4gb......not sure there is such a thing as a 8gb....I could be wrong. All you have to do is buy one of the old HDs like what has been suggested, take it apart and put it in the slot for the HD on our Xbox. The memory that you are using right now on your Xbox is internal. Once you put this new HD in your Xbox, you can move all the information and saves from the internal HD to this one you just put in. No transfer cable required and takes about 1 minute.
Once you put the new HD in your Xbox, follow the instructions in this link.
Move Xbox Content | Copy or Move Xbox Content | Xbox 360 Storage - Xbox.com