On Friday I finally got my Conroe CPU and popped it into the system waiting for it - no video output, though fans spun, lights flashed, drives whirred. I thought there must be a problem with the 7900GX2, maybe damaged in transit (it had a bent mounting plate), so phoned Scan and arranged an RMA, as the Intel D975xbx board was listed as compatible.

Went into town on Sat and got a cheap 7300 so I could at least start installing stuff. And guess what, despite being a new rev 304 board, it's using a BIOS from February (0618) - the GX2 graphics cards need the 1073 BIOS at least. I flashed the BIOS to a compatible one and now the card seems to work fine.

Just posting this in case anyone else gets similar problems, since RMAing the card would have been useless.