Did you set your camera to the smallest aperture it would allow for? What is the smallest aperture the IS can go to, by the way? With the Powershots it is only f8 ... and 15 seconds is the longest shutter speed I can choose on the Powershot. The IS may take you further (since it takes you up to ISO 1600, too, as you said in an earlier post).
The question about the smallest aperture is because that would make your light sources smaller, and more star-like, less like big bright blobs.
I don't know if a wall was the only support you had, so you therefore had to up the ISO once again (the first looks quite noisy). But if you get more and more interested in night photography, you won't be able to pursue it without a tripod. That would allow you to stay with your normal ISO settings, small aperture, long exposure times ... and if you fear that your releasing the shutter may give the camera some shake but you haven't got a remote release (I haven't, for example), then set the camera on timer and let it take its photo 10 seconds after you last touched it.
