[johnnym]

Recently i got luck and found and affordable Compaq/Digital Personal Workstation 500au.

Congratulations! A true classic.

I wonder if it's possible to use standard REG ECC SD-RAM with it?
A test with two sticks of Kingston KVR133X72RC3/256 "standard PC REG ECC RAM" failed. The memory gets detected at boot, but the system crashes shortly afterwards.
Now i wonder if it's just an issue with PC133 or that "standard PC REG ECC RAM" simply just doesn't work with the system.

I believe I've made the same experience: it starts promising but crashes in OS. But I am not sure what I have installed in my PWS 500au currently. I can have a look tomorrow, it is in storage.

When looking at the Digital Technical Journal, Volume 9, page 56, figure 7 there already seem to be buffers between the memory slots and the 21174, so maybe additional buffers on the memory modules are making problems. Figure 8 shows what looks like a buffered ECC DIMM beneath a B-Cache module - which looks like it implies those could work, too - but I believe figure 7 only shows unbuffered ECC modules installed on the MLB.

Also: Do standard radeon 7500 pci cards work in them? I have Tru64 5.1B running atm.

Not sure, mine had an ELSA one in it, but I didn't use it. Maybe it will work with AlphaBIOS running. I know that newer AlphaStations like the DS15 could use Radeons, but if the PWS's SRM can work with them I don't know. Well, you alway can access the machine via the serial console when running the SRM, so maybe just give it a try.