
Video card he go bang.
Well, more a whimper, or possibly a long drawn out scream, with two screens overlaid with pale blue spots in a pretty but unuseful way that shagged the drivers (X eating 100% of a CPU and claiming to be too busy to service anything as mundane as a keyboard interrupt). Thank various ghods for ssh and a (relatively) clean shutdown.
PCWorm have supplied a replacement generic nVidia for 80 Earth pounds which should have been it (no DVI on this one, but that just leaves me with a spare adaptor). What I didn't think to look for or, indeed, see were the big letters "PCI" on the box.
Feh.
Pull the firewire card I'm not actually using ATM and thankfully the old card was functional enough to be legible(ish) in text mode, so BIOS VGA device AGP=>PCI. All I had to do was find out WTF PCI device number the kernel thought the first slot was on, which xf86cfg (useless as it is in most other respects) told me. Quick edit of XF86Config-4 and everything seems to be as it should be now.
Still, if I get (or find in the bits box) another AGP card, I could plug the spare Philips 15" panel in and have three monitors. That's if I had anywhere near my desk to put the thing of course...