RGH'ing/JTAG'ing the xbox allows the xbox to run unsigned code therefore allowing you to play games from the hdd and swap your hdd without any restrictions
the current method of doing this is called the RGH (Reset Glitch Hack) and there are two boards which this does not work with, the Xenon board and the Corona board, the Xenon board is the original xbox 360, the Corona board is from the newest slims
There are 2 slims boards, trinity and corona. however if you buy one new, chances are that its a corona and you cannot use the hack
i would suggest getting a second hand one which was bought in august or earlier (as august was when the new board was released)
If you do go ahead with this i would suggest you buy a Team Xecuter Coolrunner
if you need any help just PM me as i have already done my Falcon console
~shut
EDIT: oh and if you do go with this, make sure you are good with soldering or you know someone with good soldering skills as the points to which you solder the wires to are extremely small and unless you are good with soldering and have a good soldering iron, you will have difficulty
EDIT:
http://team-xecuter.com/coolrunner/slim_install_wires.jpgEDIT: if you do want to RGH your xbox but dont feel comfortable with doing it yourself and you cant find anyone that is good enough to do it for you, then i will do it for you assuming you pay for the postage and the chip