depends on how much you want to spend. don't be attracted by any of the mega times magnification figures - unless you are spending thousands, 100X is gonna be about asuseable as its going to get - stability/sharpness drop away massively after that with cheaper optics
as important as the optics is a good sturdy, rock solid tripod - the field of view is so small even at 40x that getting the thing to stay still enough for photography is a nightmare
refractors have the best optics, but a refractor with a big aperture (ie big lightgathering capabilities) is daft money
if you can run to it, an 8" reflector will produce fantastic results of the planets.
I had a cheapish 6" reflector (russian, was about 200 quid), anything over 50x started to get a little too blurry for pictures, but the rings/moons around saturn, stripes/moons on jupiter were easily visible and clear - individual moon features though were phenomenal upto about 200x
t'other thing to thing about is what the tracking mechanism is like - with such a narrow field of view what ever you are looking at will be shifting very quickly across the sky, and unless you can grab it in relativeley quick exposure (half second max), the 'scope will need to be tracked in time with the moving object to keep it sharp
bottom line is, bigger the diameter of the scope, the more light it can gather, the better the result.
