Officially, the Xbox 360 only supports using Microsoft's official Xbox 360 hard drives. These are basically laptop hard drives in a special (Xbox specific) enclosure. Using a standard hard drive is not a supported scenario.
You could buy a USB enclosure and connect your hard drive that way, however you'll only be able to use 16GB of the drive (that is the maximum size USB drive the Xbox would support). I think that you can have multiple USB drives connected, so you could in theory partition it into multiple 16GB partitions and see how that works.
If you want an Xbox 360 with a hard drive, I would suggest buying the 250GB model, since this is generally cheaper than buying the 4GB model and a 250GB hard drive.