Well for architecture you typically want a few things. It depends if you are doing indoor or outdoor shots, but a wide range is usually necessary. Since you already have a 18-55mm general purpose zoom, you might want an ultra wide angle lens such as the 10-24mm Nikon. It runs about $800USD. Other options would include a telephoto lens such as the one mentioned below, or a tilt-shift lens, which would probably be quite a bit above your price range. It really depends on what you want to create as far as architectural photographs. Almost any lens can at least create some images of architecture, but only you know what result you want, and thus what lens to purchase. You may be best off replacing your current kit zoom lens with a higher quality one, with a wider aperture and higher quality optics.
If you are looking for "deep zoom" you probably want something in the range of 70-200 or 70-300mm such as the Nikon AF-S 70-300mm f/4.5-5.6 VR($600USD). That fits nicely in your budget, and gets great quality.
Another option is to jump up a bit over your price range, and get a higher quality general purpose zoom lens such as the Nikon 18-200mm f/3.5-5.6G AF-S. It would extend your current range up to 200mm, and provide higher quality while doing it.