mmmm.... since you have a D40, I'd assume you have the 18-55 which is quite wide angle and would be nice for cityscapes, unless you are looking for faster glass.
As far as Nikkor goes these:
Nikkor 20mm f:2.8 ($490)
Nikkor 24mm f:2.8 ($310)
Nikkor 28mm f:2.8 ($235)
Are all excellent lenses (the top two more so), but less wide angled that what you already have. You won't get AF on a D40 with them either.
If you want to try Sigma, this lens
Sigma 30mm f:1.4 ($429) has a VERY good reputation and will do AF on the D40, but I don't know if it's wide enough for you.
If you want wider than your 18-55, then you have:
Nikkor 12-24mm ($920)
Sigma 10-20mm ($500)
Tokina 12-24mm ($500)
Tamron 11-18mm
Both the Nikon and the Sigma wil AF with the D40, don't know about the others.
I hope this helps, and it would help if you could add some details about what you need, or why you think the 18-55 won't cut it.