Steps to take for your Xbox when you get an HDTV (assuming 16:9):
Go into the video settings
Set the aspect ratio to 16:9
Turn on each format your TV is compatible with (480p, 720p, 1080i).
Now, one final step which many people don't know is how to change the Xbox Dashboard (the menu system) to progressive. Trust me, it looks way better, but you have to have Xbox Live in order to do it. If you have Xbox Live, the automatic update has more than likely already been installed on your Xbox. The way to test it and to also activate the 480p mode is as follows:
Hold both the L and R triggers, then click both the L and R thumbsticks down. The screen will flicker and voila! 480p widescreen Dashboard. Clean lines, solid progressive image, much better color.
Also, Monster cable is usually fluff, but can see the difference between the Microsoft brand HD pack and the Monster HD pack - I have tested the two side-by-side on the same TV and the image does appear sharper using the Monster set.