Thanks for everyone's timely and very helpful advice.  I tried first the 
Twinview option of the nvidia driver, and that seems to be exactly what 
I wanted without much fuss.

All I really had to do to accomplish what I wanted was:

- download the latest nvidia driver installer ala Troy (there is a new 
version on the web since his posting)

- run "nvidia-xconfig --twinview" at which point the two monitors 
worked, but didn't seem to work at full resolution

- modify the XF86Config file so that

Option "MetaModes" "1600x1200, 1600x1200"

gave me a nice high resolution display on both monitors.

Troy Dawson wrote:
> I have to say, I'm rather surprised that nobody has said to download 
> nvidia's driver and read their documentation.
...
> There is the way that makes the monitors think they are just one big 
> monitor (although there are two backgrounds).  In this documentation, 
> that is called "TwinView".  This is nice because you can drag your 
> windows from one window to the other, or even halfway between.
> The problem with this is when you don't have monitors with the same 
> resolution.  Then things get really odd going from one screen to the 
> other when your screens don't have the same resolution.
> TwinView is talked about in the documentation, I thought it was pretty 
> good documentation for that.
...


-- 
John Haggerty
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voice/fax: 631 344 2286/4592
http://www.phenix.bnl.gov/~haggerty