I took out one DIMM and re-seated the other in
a different location as you suggested. This time
it booted and gave me a screen with the following:

The amount of system storage has changed.
aLERT! Previous attempts at booting this systems
have failed at checkpoint [1thr]. For help in resolving
this problem, please note this checkpoint and contact
Dell Technical Support.

Strike the F1 key to continue, F2 to run the setup utility
Press F5 to run onboard diagnostics.

I pressed F5
It's going through Pre-boot assessment build 4521
so far everything is OK...

Should I click F1 afterwards to continue or put the other
DIMM in and see what it does?

Thought/suggestions
Robert