Buy Xbox 360 4gb
You can actually use a Had drive from the phat xbox 360 you just have to take apart the hard drive case and pull out the SATA hard drive inside. The trickiest part is getting the hard drive lined up to properly connect into the slim console. You can connrct the hard drive without the drive caddy part but then the hard drive will just be plugged in to the plastic connectors and will hang down just a little bit inside the case but will be fine as long as you are gentle when you move your console.
buy xbox 360 4gb