I'm a little confused by your issue, but I'll take a stab at it
It sounds like there are some games you want to play at 1600x900, but the game isn't showing that as a resolution option. If that's the case, you'll have to Google whatever game it is, looking for "custom resolutions" and see if it brings anything up. If you can't find anything, then its likely that the specific game doesn't have the ability to do anything with custom resolutions (some do, some don't).
Now, if you're talking about that resolution from within WINDOWS, you can always use PowerStrip to set up a custom resolution. The only other way to do it would be manually adding the resolution to the driver's INF file, and completely reinstalling the driver. That solution, however, I don't recommend for someone not experienced with NVIDIA's INFs as its quite easy to mess everything up if you don't know what you're doing.