New 17-55IS questions and samples

Hey everyone!

I finally purchased the EF-S 17-55 IS today (for my 450D/XSi) and strolled around downtown Manhattan taking pictures. I absolutely love the lens so far (albeit I have only I had it for less than a day!) I am posting some pictures that I have questions about. Very minimal post-processing was done to the pictures, and all were captured RAW using DPP.

In the first picture, I am really happy with the landscaping and the fountain, but I had a difficult time capturing the sky and the buildings in the distance (they are 'blown-out'). In order to capture the fountain and landscape, I had to lower my shutter speed and aperture in Manual mode, but this didn't work so well for the sky!

The next couple of pictures are of the interior of a church downtown. I am actually really pleased with these, for being hand-held shots in low light, but because I used ISO 800 they have some pretty visible noise (probably not too bad if they were printed less than 8 x10.) Should I have lowered the ISO, and in exchange lowered the shutter speed? Or is this an acceptable amount of noise?

The last two shots I added because I am fond of them!

Thank you so much for your help. I am an amateur and trying to grow with these wonderful tools! Any advice is greatly appreciated.