Canon sd850 is vs sd870 is vs sd770

[svetlae]

Hello everybody,
I am thinking of buying a new camera to replace my old (4 years old) Casio.
I am liking 3 canons so far sd850, sd870 and maybe the sd770......
Can you give me your opinion on these cameras.
I am looking for great IQ, optical zoom.
Also I like the time lapse option, but can you zooom during taking the video with the above cameras. I couldn't find 100% answer to that question.
Also IS is a good feature, but they say the image gets too much noise on high ISO....
Please advise!

Regards,

Svetla