I am trying to setup dual display on a Asus EN8600GT SILENT with two DVI (with GeForce Release 175 drivers).
The problem is in the GeForce setup I can only select single, clone, horizontal span or vertical span.
What I want is DualView with two different resolutions (one for the monitor 1680 x 1050 and one for a 720p projector).

Even on an older PC with a 7300LE with FroceWare 82.65 I can select DualView and setup different resolutions.

Why don't I get an option to select DualView on my 8600GT?

You can try "Force TV Detection" as some others had this problem and solved it with that trick. You can also try different/older drivers.

I'm connecting the Projector via a DVI to HDMI adapter to the HDMI input of the Projector.
How would "Force TV Detection" help?

No idea. I just saw other people having the same problem via a google search. Two displays, nvidia options have single, clone, h span and v span and no dualview. They said they clicked "Force TV Detection" and the option appeared, regardless of the display connected. You just don't want to try it because it'll probably make you reboot.

Edit: You can also just try to enable the display through the windows display properties. I have read that some people were able to use that function. Another person was able to set it to span in the nvidia settings and then rebooted, which then displayed the option for dualview. Other than that, I don't know. Perhaps nvidia has a better answer.

For some reason unknown to me, the TV has to be plugged in when the computer is turned on. If you plug it in after you must restart to get dualview.