I realized that the problem was that I was using the VESA driver as my Generic Video Card in xorg.conf. I had looked into installing the full nvidia driver that seemed to match my video card, but that was complicated, so I reverted to the VESA driver.
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Finally, I found the NvidiaGraphicsDrivers page on the Debian Wiki: http://wiki.debian.org/NvidiaGraphicsDrivers. Here I found that Debian comes with a basic, free 2D Nvidia driver named nv.