The slim does not come with a 120GB hard drive.
The S model comes with 250GB hard drive.
There is a Arcade replacement with 4GB of flash memory, and a matte finish to it (what many are wanting).
The 4GB model will also be the only one for the time being being bundled with kinnect so many will be getting this model.
I would say just use a 2.5" western digital 250GB hard drive and make it into a Xbox360 drive (apologise if that cannot be suggested).