Well, first you have to get a mod chip. Mod chips run ~$50.

Then, set up the mod chip to run and what not.

Hook up your new hard drive, and run the boot disc for your new GUI. At this point you can FTP to your Xbox from your computer (through a patch cable). Once you patch into it, you set up the GUI to run off the HD on startup.

Then, you have to run a program from the Xbox that you ftp'd over to re-"lock" the hard drive so you can use the original Xbox bios (say to use Xbox live or something).

I made it sound easy.....and it's nothing but.

[edit] Yeah, you have to take it all apart. For the mod chip, you have to do a lot of soldering, or spend an extra $40 on a solderless mount.