The 4000 models (2U) support 4 drives. It has 2 drives per IDE cable, like a normal pc, and a power switch on the back. The 4000 will handle 4 250gig HD in a raid5, without any problem. Now if yours is a 1U, it's possiable someone swaped the MB's out. Which makes it a smaller foot print, and has a better ps in it.
The
wiki secion has pictures of the models.
As far as your AD support, that a tough one.
With all of the Vista testing going on, I see nothing to want me to upgrade. Beside requiring a 2 gig of ram to make it run smooth. I will wait a year or two till they get all of the security bugs fixed. It will take that long to prove it has security. Besides they said that about XP, and you now all the bugs it had.