That was it. I disabled my 19" and selected 1680x1050. Then i left the game re-activated my 2nd monitor and started the game and it works. I did notice some graphical glitches that weren't in the non widescreen version on the title screens. There are vertical black lines on it. I can totally live with it though. I remember a lot of games that were designed for the 3dfx boards that would do something similar due to the limitation of texture sizes back in the day.
Kismet: In my dual screen configuration, Giants seems to always use the screen marked as primary in the Control Panel, no matter which driver I pick in-game.
I'm not having problems running it in widescreen with dual-screen enabled, but both screens support the same resolutions so I can't tell if the game eventually privileges one of the displays when it comes to listing the available modes (EDIT: with a fast google search I've found some user reporting that the game lists only the resolutions available on the smaller screen).
I'd try to temporarily disable multiple screen support in NVIDIA Control Panel/ATI Catalyst Control Center and run the game with only the widescreen display active (as suggested by Klink, but without removing drivers/monitor, just using the GPU Control Panel), if you haven't already.
