Unfortunatley, the 250gb Mr N mentions will not work for the older models. I recently went to GS to buy one for my console and it only comes as an internal drive for the new consoles that came out last year. It seems some of these are sold with just a 4GB drive and the wireless already built-in. But then you can purchase the 250G drive and install it in the console.
But the 250GB can only be used with that specific model. I was told that the largest one for the older models were 120G..which I got.