I'm glad you posted this, as I'm debating the same thing. I currently have the 400D and the 50 F/1.8, but realized quite quickly that I needed a wider lens so I don't have to stand at the furthest end of my room to get more than a portrait shot. I live in the city as well and I noticed that I find myself wanting to capture some of the grand monuments or buildings here. But with only a narrow street of distance, I usually can't accomplish that without having to stitch the images together.
I can't give a recommendation. I haven't compared the difference between 28 and 17. I was hoping to grab the 28-75 and supplement the wide end with the kit lens (I hear it's a decent lens when it's stopped down). Perhaps upgrade to a 10mm/12mm super telephoto one day. And perhaps someone with more experience can answer - but doesn't one usually want a smaller aperature (higher F stop number) to go with the wide end to have a larger DOF? I guess that's personal preference, but I didn't know if F/2.8 would be nearly that useful on the wide end as it would be on the telephoto end.
Tamron 17-50mm f/2.8 XR Di II

, 135mm f/2.0
