Right, on saturday im going to buy my new Xbox 360 Slim 4gb, also Fallout 3 goty for ￡18. I noticed its 2 disc and 5gb on xbox 360. Does this mean i wont be able to play Fallout 3 Goty on my new xbox?

You won't be able to install the game, but you can play from disc. Unfortunately this means you'll probably suffer from more performance based glitches. That goes for a lot of games, and 4GB is not large enough for most (any?) games to install. It will also limit the number of add-ons and saved games you can have on your drive at any one time, though it should be sufficient to have all of the FO3 add-ons installed at once. If I were you I'd shell out the extra $100 to get the 250GB xbox.

ahh ok, il just buy xbox 360 elite 120gb.

Highly recommended!
If you don't install it to hard disk, you'll spend a lot more time watching loading screens ...