How long have you had your current Xbox for? Has it red ringed yet? I had the same one and it red ringed about the same time once a year. It will probably be worth just getting the 250GB version. Don't bother with the 4GB, once you connect to XBL it takes up most of this memory. When i worked in game (About 2 yr ago) your system was worth about £80 with 1 controller and all the bits etc. It is probably valued less now so if you are looking to save some money, trade it in sooner rather than later if your looking to upgrade.

