It was a straight swap - I've no idea why one card would fail and another one work, but the shop wanted to try that and it worked. I got a call from them saying they had tested it and it was duff, but I've got no idea why it worked - or seemed to work - in an older system.
As for the i845 frying thing, I guess I got lucky. Had no idea that sort of thing could happen. Thanks for the link
