Reset the CMOS Memory (BIOS) and see if that will help.  With the PC powered off, remove the CR2032 battery on the motherboard for about 10 minutes.

HERE is the Crucial memory listing for this model. Although other non Dell brands may work, Crucial is guaranteed to work in Dell's.  Check your new memory specs with what Crucial lists, including the voltage.

Are you sure you're seating the modules correctly? The retainer clips should snap into place automatically when the module is fully and correctly seated.

Assuming that clearing BIOS allows it to boot up with the old RAM, swap in the matched pair of new modules and then do the battery thing again. That may -or not- allow it to boot, because as fireberd said, not all RAM may be compatible.

Ron

Thanks for both of your responses.

With the power cord disconnected, I removed the battery, waited 20 min and replaced it with a fresh one.  Then re-booted with the original 2 x 256 sticks in place.  Same result.  Memory not recognized.  Re-checked the seating and tried again.  Same result.

Is it possible my proceedure did not clear the CMOS memory?  

Are you still getting GYYY?

In that case, power off, unplug, press/hold power button for ~15 sec. Remove the (old) RAM module from slot 2. Then remove the motherboard battery and press/hold power button for ~30 sec. Reinstall the battery (right-side-up!) and try it now.

If that boots, power off, unplug, etc and swap the other module into the same slot. If both work in slot 1, add the other one back to slot 2, and try again.

Ron