[astro744]

An 8" Aperture is more than enough to provide plenty of detail on planets provided it is not grossly out of collimation. However being Dob mounted makes it more difficult to concentrate on the detail since you are constantly nudging the 'scope.

You could invest in a couple of ultra wide field eyepieces to keep the planet in the field longer but for the cost of these you could also get an EQ mount. Whatever eyepiece you use must be of reasonable quality and most these days are quite satisfactory on axis but better can of course be bought usually to provide wider and sharper off axis views

Jupiter is getting too low now for serious observation as there is too much of the Earth's atmosphere between your telescope and Jupiter making the seeing very poor.

Saturn has close to edge on rings at the moment and some subtle surface banding but not to the extent of Jupiter's festoons, countless bands and other intricate features.

Mars is quite small this opposition and you will have to wait until 2018 for it to be almost twice its current size. However even now the polar cap and dark band surrounding it is clearly visivble in a 4" refractor and 6" reflector and should be quite evident in an 8" reflector. Syrtis Major too is quite obvious when facing our direction.

A 12" telescope will show you more detail when seeing permits but as is often the case a smaller aperture sees less of the air and therefore less of the turbulence and can give you a better image more often but with less resolution than a larger instrument. (Depends on the size of the steady air pockets).

A lot of perseverance and a bit of patience will soon have you seeing lots of detail.

Best magnification is 120x to 240x and more if seeing permits. Jupiter is quite large enough to see detail at 120-150x but Mars will need 160-240x.