4GB Xbox

If you wanted to add the hard drive at a later date you'd just need to plug it into the console. Do be aware that hard drives for the slim aren't compatible with the fat consoles and vice versa.

Yes the Xbox 360 slim just like the old console models have changeable HD's you would just need to transfer your save data and the like via a Data cable or USB. As stated before Fat and slim xbox's do have different HD's and they aren't compatible.

I bought the 4 gb model, and also got the hard drive at the same time (only cause the hard drive was on sale and i got a free controller with it :) ). I also like that its the matte finish cause i move my xbox around A LOT. i like having the extra 3.x gbs, even though i have 184 gbs left on my hard drive. To be honest though, i would maybe buy the 250 gb model. your gonna want it, especially if your going to get downloadable content, such as map packs. you may save cash now, but your going to spend another 20 bucks down the road on the hard drive. i would but the 250 gb system and go buy some used games from gamestop and get the power up rewards (cost 14.99), cause you save 10% on all used game purchases. If you have any questions, feel free to ask me.