system-config-display is borderline deprecated these days, and doesn't understand RandR 1.2 anyway, so it's not going to give you any joy.
Editing xorg.conf based on the NVIDIA driver's settings won't help, because NVIDIA uses its own proprietary multi-monitor system (TwinView) and hence the parameters which set up the NVIDIA driver correctly for a multiple monitor case won't correctly set up *any* other driver.
You could happily run gnome-display-properties in Xfce and it would likely work, but if you don't feel like installing it, the easiest way to do things would be just to use the command line xrandr tool.
Just running it will tell you what displays are connected and what they're capable of, thusly:
$ xrandr -q
Xlib: extension "Generic Event Extension" missing on display ":0.0".
Screen 0: minimum 320 x 200, current 1280 x 1024, maximum 2048 x 2048
DVI-0 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 380mm
x 300mm
1280x1024 60.0*+ 75.0 60.0 60.0* <=====
1400x1050 60.0
1280x960 60.0
1152x864 75.0
1024x768 75.1 75.0 70.1 60.0
832x624 74.6
800x600 72.2 75.0 60.3 56.2
640x480 75.0 72.8 72.8 75.0 60.0 59.9
720x400 70.1
VGA-0 connected 1024x768+0+0 (normal left inverted right x axis y axis) 430mm x
270mm
1680x1050 60.0 + 60.0
1400x1050 74.8 60.0
1280x1024 75.0 60.0 60.0
1280x960 60.0
1152x864 75.0
1024x768 75.1* 75.0 70.1 60.0 <========
832x624 74.6
800x600 72.2 75.0 60.3 56.2
640x480 75.0 72.8 72.8 75.0 60.0 59.9
720x400 70.1
This is just some random output I stole from a site somewhere, as I'm running the NVIDIA driver myself so I can't show you mine.
With the above output, you could do this:
xrandr --output VGA-0 --right-of DVI-0
and that would give you a dual monitor setup with the monitor plugged into 'VGA-0' to the right of the monitor plugged into 'DVI-0'. (Actually, that wouldn't work for the unfortunate person whose xrandr output that is, for icky reasons I won't go into here, but happily, they shouldn't affect nouveau). It's usually quite easy to figure out which 'output' (in xrandr's parlance) translates to which monitor on your system, but if it's not easy in your particular setup, just guess, and try again if you get it wrong. Nothing will explode, you'll just wind up with the displays reversed from what they should be.
You can set things up in xorg.conf for RandR 1.2, so you don't have to run xrandr every time you log in (or just script it up). If you use gnome-display-properties in GNOME, it automatically sets to the layout you select each time you login (though, of course, GDM or whatever won't use the same setup). If you want to do it in xorg.conf, refer to:
http://wiki.debian.org/XStrikeForce/HowToRandR12
It would be great if you could switch back to nouveau for a bit and see if this works - we do want to make nouveau as good as possible, and dual monitor setups are one area that definitely needs testing. (Mine, unfortunately, really doesn't work - nouveau just can't cope with my card having two monitors plugged into it, it hangs on startup - which is why I'm using the proprietary driver).
Linear Mode
