Also, I'm on x86-64.
Also, I have TWO NV graphics cards in this system.  The problem occurs only for
the one plugged into the slower PCI channel.

Also:
[    0.256789] pci 0000:02:00.0: Boot video device
[    0.256791] PCI: CLS 32 bytes, default 64
[    0.256980] PCI-DMA: Disabling AGP.
[    0.257075] PCI-DMA: aperture base @ b4000000 size 65536 KB
[    0.257079] PCI-DMA: using GART IOMMU.
[    0.257083] PCI-DMA: Reserving 64MB of IOMMU area in the AGP aperture

so the device having problems is NOT the boot device. strange, because this is
the second device detected by nouveau...

Also: first nouveu error is during boot:

[    5.084189] nouveau E[   PFIFO][0000:01:06.0] DMA_PUSHER - ch 0 [DRM] get
0x00000000 put 0x00000188 state 0x80000000 (err: INVALID_CMD) push 0x00000000

and occurs before the second card is detected. No further problems until about
3 minutes later, after the first attempt at Xorg has failed, and the second
attempt to start X gets going.