the 8" dob and t-ring will give a wide field of view which is great for the moon but Saturn and the planets will be quite small. to get some more 'zoom' a lot of people use this combo:
http://www.bintel.com.au/Eyepieces-a...oductview.aspx
http://www.bintel.com.au/Eyepieces-a...oductview.aspx
5x can also be used for visual use too with eyepieces.
a good way to do planetary with a non tracking dob is to record in video and guide manually the dob by looking through the eyepiece. smooth movements are required though!
if you think of getting into deep sky photography you could probably mount your existing dob tube onto a german equatorial mount - something like a HEQ5 or NEQ6.
cheers