NP. I'm sure you can appreciate we'd only be guessing.
You could also try posting in the Dell User forums about your mobo AND your BIOS revision and asking for anyone who has installed more memory successfully. But it seems their forums have been more watered down than usual lately as more folks build their own.
Thanks. Iwill try dell user forum, but I don't expect much since there appear to be virtually no questions re xps 420 platforms, and those that are refer to the older quad 6 series processor.