Courier duly handed over the replacement motherboard today, and took away the faulty one. The board is marked as refurbished, but looked less dusty than the one that went away.
I took it carefully, putting it in, connecting power only (3 power related connectors) and then fired it up. So far so good. Then memory etc. Beeped away.
Then all the various connectors, and the old and original graphics card. Booted up fine and I checked the BIOS - interestingly the asset tags common to Dell systems where absent. No mention of XP reactivation - woohoo.
Then removed the graphics drivers, rebooted, shut down, pulled the old graphics and added new graphics and sound.
Now running with an Nvidia 7600GT, which replaced a 6600GT.
All hunky dory. I'm really pleased with the service I received from Dell, their online hardware support chat service and the engineer at the other end were both excellent.