Looks like a mix of the dry air in the winter, bad heatsink design, and heavy hitting SL sessions on a dual-link DVI 30 inch monitor, has toasted my X1900 PCI card in my Mac Pro. When it starts at all, I get a flickering field of duff squares instead of a picture - and when I drop in a replacement card, I get straight back to normal operation.
Apparently the X1900 is well known for being prone to fluff clogging, due to the heatsink being shallow and packed tight with vanes, and it has to move a lot of heat - and the speed-adjustable fan doesn't help t all. To do that it would have to puff in reverse!
So if you have an X1900 in your Mac Pro, open it up and show it some luuurve, or face having to fork out some dosh for a replacement - there are a good few around, though most seem to be re-flashed PC ATi cards and therefore something of a hostage to Software Update...