Check the specs,

* SD890 IS - 10 megapixel sensor, 5x optical zoom, 2.5-inch display, sub 1-inch macro
* SD790 IS - 10 megapixel sensor, 3x optical zoom, 3-inch display
* SD770 IS - 10 megapixel sensor, 3x optical zoom, 2.5-inch display


I'm going to risk the SD890 because I think I would miss the 5x zoom too much... But my opinion would be to go to the store and compare the two or reading some expert reviews will always help.