RAID Key not detected

That should work.  What kind of memory did you install?  This is not regular system memory - should be spare part# G5555.

The memory is a 128MB 186 pin 3.3V SDRAM DIMM, unbuffered, ECC, 100ms, CL2, 16Mx72 (actually Kingston KVR100X72C2/128). This should be practically identical to the Micron Part that Dell ships as G5555 (I found the specifications from a posting by a Dell employee in this Forum).
Shouldn't the controller simply act as a RAID controller without write caching, if memory or battery are not present?

I have the same problem. Tell me if you manage to fix that and how you did it.

No, I have not been able to fix this. I have tried another Key with the same result. Dell Support seems to think this is not their problem as we have no support contract. If the mainboard is broken, it will propably be easier to buy a third party controller than to replace the board. Considering Dell's spare parts policy I am a bit reluctant to do anything that forces me to rely on their help.
I'm running out of options in this issue ...
Guido.

i still haven't solved my problem but i'm sure that in your case could be the same problem that i have, the memory is not good. I will order G5555 from dell. is not that expensive, around 75$.

I would be very interested to hear if that has solved your problem! I have not found exact specs for the Dell memory module, but Dell technical support said my module should be ok (100MHz unbuffered ECC).