More RAM, installed.
Dell support came through for me in a matter of minutes. About 45 minutes after I e-mailed them from support.dell.com, I received this:
If you upgrade the memory, the RIMMs must be upgraded in matched pairs of identical MB capacity in both sockets 1 and 2 or sockets 3 and 4. Be sure to install RIMMs in the first two sockets nearest the processor before installing RIMMs in the outer two sockets. RIMM slots without memory modules need to be populated with continuity.
So I installed them in pairs - and whalla!