I haven't heard much on the Digital Signature, but I do know Longhorn will have it. The Longhorn installation process does take a LONG time, and it doesn't match up with the estimated times given; for example Gathering the computer's information was suppose to take 20 minutes for me, but it really took 35-40 minutes, and detecting hardware took about 50 minutes or so.
My system specs:
AMD Duron 750Mhz
768 Mb PC 100 Sdram
Used a 4Gb hdd I found on the side, and it left me about 200-300Mb of free space after the installation of Longhorn.