From Tim Murphy of Dell:

Some of the main technical differences, eg:
- DRACIII (6G RAC) has no coresident BMC firmware (RAC and BMC have
separate FW images, H/W i/f's). console support via VNC, Java applet.
support for battery, separate power supply. IPMI 1.0 based.
Local/OS/agent interface: PPP, IP addressible.  DRACIII has a few
sensors of its own (separate from server sensors).

- DRACIII/XT (7G RAC) has coresident BMC FW running onboard (requires
ribbon cable connector to main chassis). No battery, nor separate power
supply. console, IPMI support, Local/OS/agent: all similar to DRACIII.

- DRAC4: (8G RAC) no coresident BMC (separate FW).  no main chassis
sensor support thru RAC.  No battery, separate power supply. Only RAC
w/Virtual media support (to date). Console: superior Virtual KVM support
(no VNC, but still a Java applet). IPMI 1.5 support. Local agent
interface: raw serial (no PPP), requires OS agent running for host to
RAC commo.

And I'm sure I missed some others.. so yes, there are quite a few
technical differences across the RAC H/W and FW generations.  It is also
*not* the usual case that more than one DRAC H/W is supported on a given
Dell server (i.e. the DRAC offerings are often closely tied to their
server generation counterparts).  So a good question is also what DRAC
offering is appropriate for a given Dell server model.