The worlds briefest but still comprehensive introduction to the Alpha:

The Alpha (aka AXP) processor family was DEC's path in moving away from the
VAX and PMAX (MIPS RISC) architectures to a new RISC family that could run
VMS without major architectural changes and also support Ultrix/OSF/Digital
Unix.  The first processors announced offered downright shocking processor
clock rates and performance, though other workstations and now even Pentium
based PC's have since surpassed the original performance figures.

The Alpha processors basically fit into two families, the original 21064/21164
family and the low-end 21066/21068 family.  The 21064 family has an uncommitted
128-bit data path to memory and cache.  DEC has implemented quite a range of
system designs using this family, with both PCI and Turbo-Channel expansion,
and also multi-processor server systems.  As CPU performance has moved along,
DEC has enhanced the original design with higher clock speeds, on-board L2
cache and 4 vs. 2-issue superscalar processor core.

The 21066 family is a lower end version intended mostly for use in personal
workstations.  In contrast to the 21064 it has only a 64-bit path to external
memory and cache, with a built-in PCI bus interface.  As a result of the
narrower data path, performance is about 2/3 that of the 21064 at the same
clock speed.  DEC also hasn't done as much to enhance the speed or processor
core and newer DEC systems are based on the 21064/21164 family with with PCI
bus support chips, rather than the 21066.

In the systems front, the 21064 based systems are common in the workplace and
available though used workstation channels, but much of the recent interest
in the Alpha family stems from the availability of the Multia/UDB systems and
AXPpci33 motherboards in surplus channels at increasingly attractive prices.

The Multia/UDB (same hardware, different software bundle) and AXPpci33 are
based on the same 20164 reference design - the AXPpci33 is a "Baby AT"
motherboard with PCI/ISA expansion slots, while the Multia/UDB is a very
compact desktop system, with at most one PCI expansion slot, but with Video,
Ethernet, PCMCIA slots and Audio built-in and also a SCSI hard drive and
optional floppy drive.

It is important to realize that, in today's terms, these systems are
comparable in performance to mid-range Pentium based PC's - they benchmark
better than many more expensive systems on the used workstation market,
but don't expect miracles in comparison to Pentium-Pro 200 MHz systems.
Both systems are available with 166 MHz and 233 Mhz Alpha chips, but
the 166 Mhz systems are much more common and 233 Mhz ones command a
substantial price premium.  So far, no source of cheap 233+ Mhz Alpha
chips has turned up for upgrades.

Which system to select depends on your intended use.  The AXPpci33 boards
are cheaper and allow more peripheral expansion, but you must add case,
power supply, cache chips, memory, SCSI drive, ethernet and a video card
to get you near the working equivalent of the Multia/UDB systems.

The Multia/UDB systems come in several models.  While the box supports
one PCI slot, in the systems that have an internal 3.5" hard drive the
PCI card area is blocked by the hard drive.  The SCSI controller is
on a riser card that plugs into the PCI Bus on the mainboard and some
versions of the riser don't make provision for an external SCSI bus
connector or for the PCI expansion slot.  Some systems come with 3.5"
2.88 MB floppy drives, but note that a low-profile note-book style drive
with attached ribbon cable is used, rather than a standard PC style
floppy drive.

VX40 - These are the low-end systems, with a 166 MHz Alpha chip soldered
       in and 256 K-byte cache soldered in.  The variants with no floppy,
       no PCI slot and no external SCSI connector seem to be the most common.

VX41 - These are the mid-range systems, most with socketed 166 MHz Alpha
       chips and 256 K-byte of cache in a SIMM-socket.

VX42 - These are the deluxe systems, essentially the same as as the VX41,
       but all models have 233 Mhz Alpha chips and 512 K-byte cache.

VX51 - This is the Pentium based member of the Multia family, and not
       an alpha at all.