The Dell powers up ok, spins up the drives, flashes in the 3 diodes
(caps lock etc. etc.) and then.... nothing.....

That's not good . This indicates that either the memory has failed or
the CPU , or the chipset.

Maybe you could try removing the memory sticks one by one and see if
it starts to work each time .
[...]

Sorry, but it didn't lead to anything.

If I disassemble the Dell - is there any way to measure, if any of the
problems you mentioned, is the case?

Henrik