comparing the Canon XTI/400D to the Nikon D80 -autofocus

Well basically the optical image stabilizer does exactly what the name says, it will stabilize your image through shaking or fast moving action (mostly the first, somewhat the second). So, basically it will give you sharper, crisper photos, which also leads to better contrast, etc. Now, this may and may not make a difference to you depending on which lenses you use. All of the newer digital lenses have an optical image stabilizer built in, so you don't need one in the body, hence why the xTi/400D does not come with it (neither does its successor the xSi/450D). If you use any of the older manual lenses or some of the cheaper digital lenses, they will not come with a stabilizer in them, so you will need it to be in the body if you would like to have it at all. Basically if you're buying new, good quality lenses you don't need necessarily worry about your camera body not having the stabilizer. If you have older lenses, or want cheaper lenses (including used older manual lenses), go with the body that has the built in stabilizer. Also, I'd recommend checking out the new Canon xSi/450D, as well as the D40x (definitely not as fancy as the D80, but cheaper and more comparable to the xSi, as it doesn't have an image stabilizer either).