Are there differences in these two cameras besides the IS and the MP? I am trying to decide which one I should get. I don't plan on doing large prints, but I may want to crop out sections and blow them up. I just wonder how much the IS is going to be necessary for a camera with only 3.8 optical zoom? Any ideas that might steer me towards one of the other?

Just to add another question? How do I find out if a camera will take advantage of faster memory cards, like the Sandisk Xtreme 2 or other faster cards?

Here is a side by side:
http://www.dpreview.com/reviews/compare_post.asp?method=sidebyside&cameras=canon_sd800is%2Ccanon_sd900&show=all

If it were me I would ignore the number of megapixels, and look at the features from there. Unless you plan on a lot of cropping and/or printing 8x10 or larger all of the time, you won't see a difference in 7 to 10mp. The IS and 28mm wide end catch my eye for the 800IS right away. And the slightly higher frame rate and the higher resolution movie mode on the 900 catches my eye. For me IS is more important than a higher resolution movie mode, and I would rather have a 28mm lens than a 35mm lens on the wide end. So for me it's the 800IS.
As for the high speed card, I know my S2IS is best served by a medium speed(66x) card and has similar specs to both cameras.

Thanks for the info. That will help me decide for sure.