I took a shot of my office, made a crop of the air condition switch and part of the whiteboard. It's quite a big difference, not only because of the higher resolution but also in quality.
Ixus 50, crop 100%
Ixus 850, crop 100%
Ixus 50, crop+resize
Ixus 850, crop+resize
The image quality is a lot better on the 850. The display on the back is also more usable. The old camera had a better casing though - solid metal, and really tight and compact. The new 850 has a plastic feeling and is clumpsier.