I have used the 17-50 non IS from Tamron on my 400D for a year and it is a superb combination. Since my girlfriend is studying arts history (hence the rapid identification of Neuroanatomists image sample [:P]), I have visited, and photographed quite a few buildings, especially churches in most cities we have visited. The 17-50 is not wide enough for indoor architectural pictures on a crop camera. I use the 11-18 Tamron for these shots, but if I could chose again, I would go for the Tokina 2.8 instead. I believe it has better build quality, it is faster, which is very important for indoor shots, especially since the 400D has substantial ISO noise from 400 and upwards.Moreover, my Tamron show substantial chromatic aberration in the corners.


My low-budget choice for architectural pictures in narrow locations would be the non-IS Tamron 17-55 2.8 and a wide zoom like Tokina 11-16 2.8. I don't know if this is within you budget, but if you get these, you will get two great lenses that will last for years. It would of course be nice with a prime, those are generally the best, but in narrow locations, there is often litte space to move around to fit the building into the frame. It is a balance between versatility, price and image quality.