Matti, The ferryride up the Bosphorus is well worth doing. But you do have another option to the 6 hour roundtrip ferry. Tur-Yol offers a faster trip--one and a half hours complete. You don't get off at the end, you just stay on the whole way. They leave more frequently than the main ferry, too, from the same docks. Just a tad pricier--when I last did the cruise a couple years ago I think there was a 2 lira per ticket difference. This may have changed, but it is probably still a negligible difference.
The best sights are within the first 20 minutes or so of leaving Istanbul, so have your camera ready for the first and last bit of the trip.
Also, keep in mind that there is no narration on these ferries, so be sure to have a written guide along with you so you know what you're seeing.
Regarding Dolmabahce Palace: It's a Western style palace built in the 19th century to impress Western dignitaries with how "modern" the sultanate was. You have to visit it in a group with a guide. I have to admit not being terribly impressed with either the guide or the palace. Others may differ, though.
Finally, "tom" keeps posting to this site here advertising websites selling tours. It's against the rules here to post ads, but "tom" doesn't seem to care. Buyer very much beware...