Digital discontinued their "Universal Desktop Box" systems in the summer of '96, which put about 8000 UDB systems on the market, initially at the not-overly-princely sum of $800. (Original cost was about $4500; these are seriously overengineered little puppies.)
They were designed with essentially the same purpose as today's Network Computers; they are:
Relatively small/compact units;
Have limited local disk resources; users are really intended to store data on servers elsewhere;
Don't have really powerful CPUs, anticipating that major applications will run remotely.
Shortly after these models were discontinued, Oracle took up a not-dissimilar vision, pushing NCs. At $800, the Multia would have been a decent seller, being significantly more powerful (at least in concept) than Oracle's NCs. Unfortunately, that $4500 cost leaves it about ten times too expensive...
I have since seen price quotes as low as $450. Vendors of course quote the original $3500 price tag as the number to compare to.
There may be a few new units around, but all that seems to be left is the secondary market...
CPU upgrades: The CPU and cache are soldered in (i.e. you can't upgrade).
The unit was designed as an X-server, so presumably there has been some effort put into making the graphics hardware fast. It seems pretty quick, with the downside that it only supports 8 bitplane mode.
Memory upgrades: There are 4 slots, currently they are fully occupied by 2x8M and 2x4M SIMMs. Any upgrade will require removing some of these SIMMS. At the trade-in price for SIMMS, you may as well keep them and put them in any other machine (they are standard JEDEC 72 pin 70nsec SIMMS).
There is a space in the case that can accept either a short PCI card or a 3.5" hard drive. Size is a factor; it may need to be a very short height (1/3 height?). Note that there is enough x86 emulation on board the CPU that it is possible to boot up with bigger and better video cards that assume you have an x86 CPU to run onboard BIOS code.
Eric Smith has collected a lot of information about Digital UDB Board Jumpers which should prove useful if there is need to reconfigure the hardware.
There are two versions of the motherboard - one with soldered in 166mhz CPU and cache and one with a ziff socket and the ability to run up to 300mhz parts (even though they were never released), and a user upgradable cache (from 256 to 1meg). The second motherboard is also able to accept some flakey 12meg simms in the first 2 memory banks.
For Alpha assembly aficionados, documentation was collected for the NetBSD Project that may prove useful.