Its in the nVidia Control panel. Its all a lot easier than I think you suspect.
None of this has anything to do with Matrox yet as you dont have a TH2G.
To test 1680x1050 57hz (ie to see if your monitor will work in 5040x1050 triplehead 57hz) simply do the following:
Right Click on Desktop and select NVidia Control Panel.
Under the Display drop down mention on the Left select "Manage Custom Resolutions".
Put in 1680 (Horizontal value) and 1050 (Vertical), change GDI refresh to 57. Click on "Test".
Thats it.
To actually Setup 5040x1050 57hz, do the following:
Right Click on Desktop and select NVidia Control Panel.
Under the Display drop down mention on the Left select "Manage Custom Resolutions".
Select Create, put in 5040 (on the Horizontal value) and 1050 (Vertical).
GDR Refresh set to 57, bits per pixel 32.
Then click on "Advanced"
In the Back-end Parameters, Timing Standard section change it to
CVT Reduced Blank first, then select
"Manual".
Setup the
Manual timing as per this image: