If you modded parts to your xbox you wont be banned (but some cooling systems ban, not sure why). if you install CFW you WILL. No doubt.
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Dude I think you are getting ripped off. Go mod it yourself. Buy a 320 GB Western Digital HDD and mod it using HDDHackr. You could buy a 120 GB HDD, but with a 320 GB if Microsoft released a bigger HDD like 250 or w/e you could easily upgrade to it. After you mod your HDD using HDDHackr just follow my tutorial for modding games. Only instead of CodTool which is only for Call of Duty 4 and Call of Duty World at War, use Modio.
YouTube - Call of Duty 4 360 Hacks
I would get it
Why spend the money when you can do it yourself. Plus you'll learn what to do and it'll be helpful if you decide to go further in your xbox hacking escapades.