If you are very serious about doing architectural work, the 24mm PC Nikkor is great. It covers FX, but is still fairly wide on DX. Here is a shot from literally across the street from this building. Some Lightroom 4 correction was also applied. The corrections post processing are very good, yet I am a believer that the original is always better.
D4, 24mm f/3.5 PC Nikkor
Post Script: Well, I went out and shot a couple on the D90 with the 24mm PC Nikkor. And, now I know why they have all the warning about pinching fingers. The bodies with built-in flash hang out on the front enough so one can easily pinch the fingers. Thus, allow me to correct myself and NOT recommend the 24mm PC Nikkor for the cameras with flash. Instead I would suggest the AF-S DX NIKKOR
10-24mm f/3.5-4.5G ED as the one which will give the widest view. (An actual horizontal 91 degrees or vertical if portrait position)