Uhm, GRAW has native widescreen support. If 1680 isn't showing up it isn't the game. Is 1680x1050 what your desktop is set as?
Heh, GRAW is stupid however. Hoose was right; if you don't have a 512MB card you aren't presented with the higher resolutions, which includes 1680x1050. The idea is to ensure the player cannot select settings that are going to overload the system, but completely ignores advanced users and situations like this. It's completely retarded, but then again it's a crap engine.
Hoose, you have to got to the "Ghost Recon Advanced WarfighterDatasettings" folder (where you installed the game) and edit "renderer_settings.xml". You'll find a resolution entry there if I remember correctly; just change it to be 1680 x 1050 in the way it is already set, and it will be all good.