Before you jump feet first into upgrading the HD and changing the softmod, there are a few things you should do.
First, FTP the C and E drives to the PC for a backup. Make sure the eeprom.bin file is included. It should be on the E drive in a temp or back up folder.
Second, play around with the Xbox to learn what you can do with it. You may like XBMC as the dash.
Third, check to see if the Xbox can be TSOP flashed. If it can, you can put in your hard drive and use Slayer's to build a new HD with just the Xbox.
Before you begin all these steps, take some time to do some reading here. While you wait on your Xbox to arrive, this would be a good time.
There are many people here that have problems because they move too fast and don't back up their existing files. The eeprom.bin file is THE MOST IMPORTANT file for a soft mod. With it you can build a new drive from scratch and lock it to the Xbox it came from.