This camera is nearly three years old, which is ancient in digital camera terms. We recommend that you buy one of Canon's newer, cheaper, and more powerful SD series Powershots. Check out the SD1300.
Here's what we had to say about the SD850 when it was released in May 2007:
What more can be said about Canon ultracompacts? People love them, they're sleek, stylish, and chock full of great features. The SD850 IS is no exception. Building on the popular SD700 IS, the 850 retains the vital image stabilization that makes shooting clear shots with a small camera a breeze. On top of that, the 8 megapixel sensor will allow you to print photo-quality images in sizes up to 16" x 22". Printing, sharing, and transferring photos is made even easier by a cute little button that will hopefully alleviate some of the headaches and annoyances associated with moving your pictures from the camera to your computer (and beyond).