1. I swapped out the 1Gb DIMM for 2x512 Mb DIMMs and got the same symptoms: YGGY.
2. F2 is the key to enter BIOS, not F1. (Pressing F1 at startup does nothing, and the Dell documentation states it should be F2.)
3. Pressing F2 takes me to exactly the same symptoms as (1). No BIOS, No boot, just the YGGY diagnostic lights.
4. The only recovery for this is to power down, remove the power for 10-seconds or so, and then reboot with the original memory in place. The the O/S comes up OK. (I think the BIOS must be stuck in quick boot mode, as no Dell splash screen appears - just Windows starting...)
I'm coming to the conclusion that this is a BIOS issue: Firstly, I can't get into BIOS, and secondly the Dell documentation suggests that if the hardware has change (ie, new memory configuration) it first checks the BIOS and then if recognizing a change, takes you to BIOS to configure the change.
My next step is perhaps to reset the CMOS and see if that allows me to get into BIOS. (My gut feeling is that if I can get into BIOS, I can get the new memory to work.)