I'd go for the highest widescreen option they have, otherwise the picture will either be distorted ("short and fat") or "pillar-boxed" (black bars at the sides) depending on your monitor and/or graphics card configuration.
In many cases, it's possible to persuade games to run in resolutions they don't offer in their settings, either with undocumented/poorly-documented command line options (for example, Homeworld2 can be run at 1920x1080 by appending "-w 1920 -h 1080" to the command in its shortcut) or by editing lines in their configuration files (I think the original Unreal was like this - "FullscreenViewportX=1920", "FullscreenViewportY=1080" in Unreal.ini - it's still present in the newer titles but not necessary as they offer the resolution in their in-game settings).
Judicious use of Google (other search engines are available
) may help you find tips for specific games, or you could just rummage around looking for likely files to edit in Notepad. Make sure you back them up first, though...
Hope this helps,