Yes, what you see through the viewfinder will be what is in the image.
That's the simple answer - at the risk of introducing unnecessary detail, the viewfinder on the D90 shows about 95% of the final image because of the way it's designed. That's nothing to do with the lens or DX / FX, though - the D700 which is an FX camera does the same sort of thing.
I only mention the 95% because if it's important that you capture exactly what you see (and because someone would correct me if I left it out) then using Live View on the rear monitor would be better as that shows precisely 100%.
For a wide lens to complement the 35-70mm, you'd be looking for something with an upper focal length around 35mm - and that applies whether you'd be using the lens on a D90 or an FX camera. Maybe your best option would be the AF-S VR 16-35mm f/4G Nikkor.
However... if you really never intend to go to the FX format (and many people don't) it may be an idea to look at a single lens that could replace the 35-70mm and cover the wide end. In that case, I'd recommend looking at the AF-S DX VR 16-85mm f/3.5-4.5G Nikkor, which would go as wide as the 16-35mm, AND a bit longer than the 35-70mm.