Pete,
a little bit more info would be useful.
What is the maximum resolution this monitor can cope with ?
Maybe 1920 is the max, so I would stick with it (XRes).
The YRes is personal preference, maybe 768 is sufficient.
No problem with the appropriate settings in RichardBurnsRally.ini and FixUp (fullscreenWindow=1, fixAspectRatio=1,centerMenu=1 ect).
This will enable you to see a wider scenery, useful in sharp corners.
You should close all other Desktop windows and set the Windows Desktop background to single color, black.