More than 60x per inch of aperture is sometimes possible for planetary observing with small aperture telescopes (under 4"-5"), since a small scope looks through less of our turbulent atmosphere than a large one and therefore is less affected by unsteady seeing. Unfortunately, the exceptionally good seeing that allows such high magnifications is very rare.
During average seeing conditions (the kind you find nine nights out of ten), 25x to 30x per inch of aperture is a more sensible power for binary star and planetary observing. It is at this power that the resolution of a scope matches the resolution of your eye and images are sharpest. This gives you a highest useful power of about 200x to 240x with an 8" scope on an average night, 100x to 120x with a 4" scope, etc.
Globular clusters and the smaller nebulas are best at about 12x to 15x per inch of aperture, while 8x per inch of aperture is usually best for finding galaxies and observing large nebulas.
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