Lower is usually better. If you can shoot at 100, then that is better than 200, 400, 800, etc.

But some cameras have very good performance at higher ISOs and you can make great prints at ISO 800. The Canon CMOS DSLRs are good at this.

Some other cameras have sensors whose native ISO is actually 200. In those cameras, the camera produces the ISO 100 electronically, so switching to 100 ISO doesn't give any more detail than 200. Most Nikon and Sony/Minolta DLSRs fall into this category.

With smaller sensor, point and shoot type digitals, the noise is usually unacceptable beyond ISO 200 anyway.

Of course, the noise produced by high ISO can be reduced by proper exposure. Underexposure produces more noise than overexposure. So if you underexpose a shot and correct it in photoshop, it will be noisy. If you overexpose it and correct it, it will be cleaner. That's why many experts recommend to "expose to the right" of the histogram.
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Chris Harrison
San Antonio, Texas

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