For the planets Venus, Saturn and Jupiter, you can get a good look at 50x-100x. Most any scope with at least a 3"/75mm objective and a long-ish focal length (F8 or longer) will work. For other planets (Mars, Uranus, etc) you need a lot of power to see anything more than a slightly colored blob. Well, Mars is kind of on the edge. Good seeing conditions, and 150x and it starts be an orange ball with some light and dark shadows. My 80mm scope can hit 150x, but at that magnification, the mount becomes super important - if you are on the driveway, some one can walk by and it will shake your view! My mount is a decent one - a Vixen Porta II - but my 80mm scope at 150x is about all it can handle.
A new cheep Chinese 8" dob will put up a great view at 150x, but the problem becomes tracking with the dob at that high power - the planet will only stay in view for about 30-45 seconds, as the earth turns.
Looking at planets at high power is where computer controlled scopes are great. On the rare occasion I can get my ETX90 to work right, it is wonderful being able to watch a planet for several minutes at a time. If you want a computer controlled scope, Don't cheap out! Low end versions have the worst combo of cheap, shaky mounts and cheep drive trains that either wear out quick, or are not repeatable, or both.
What Price range are you looking at?