The 256-512 hack has been released ....a few times now
Technically- no, you don't have to flash the dvd firmware to do or operate the jtag hack. HOWEVER it certainly makes things like the initial loading of XEXloader a LOT easier, otherwise you will be stuck loading xexloader onto your hard drive via PC.
*I'm not sure if the new dash released this week (the FTP one) will allow you to FTP xexloader to the console hard drive or not?) I still haven't made it to using the new dash with FTP support. Still playing catch up in m life/business.
Got a new bag tonight and decided to take the night off, maybe I'll do it tonight? probably not.
Yes, ANY
NEW xbox360 made before June 18, 2009 is a GUARANTEE to be Jtag-able. And an easy dump drive, well assuming you are brave enough to make/purchase a spear.
EDIT- Jtag console removes the "authentic Microsoft hard drive restrictions" I'm using a normal 2.5" 500gb Hitachi hard drive, I pulled out of a dead laptop. Without any alterations done to it, other than simply formatting it in my NXE dash.