Ive been busy so I havent had time to post answers on this board recently, but I had to reply to this one :)
First off, I will assume at least one of you is Catholic so you can get married in the church. Secondly, you will definately want to get married in the old town. There are several churches in the old town, but almost always weddings are in the cathedral. Google "dubrovacka katedrala" and see images of the church. Unfortunately, their website was suspended so this is the only way to see those pics right now. Here: http://free-du.htnet.hr/Mala-Braca/mapa.htm is a map pointing to another church - Mala Braca, the franciscan monastery (number 2). Number 12 is the church of st. blaise, probably the most famous Dubrovnik church. 15 is the cathedral, 9 is the Jesuit church, 13 is the dominican monastery. Of these, I recommend the cathedral because it will be the most simple to organize and be allowed to have it and the church is large and airy (for August wedding!!!) Also the franciscan monastery (http://free-du.htnet.hr/Mala-Braca/) is beautiful, but difficult to get an ok for the wedding... believe me. For more pics google image words "dubrovnik cathedral". For st. blaise church try "st. blaise dubrovnik" and "sv. vlaho dubrovnik".
Also, how many people will be at the wedding and, if it is many, have you considered hiring someone to help you? You will find this thread most helpful: www.dubrovnik-online.com/forums/index.php…
If it is just you and your fiance then I imagine you wont need much assistance, although it may be nice to have someone help with the arrangements. The thread has lots of recommendations.
For hotels, I suggest Hotel Argentina, Hotel Dubrovnik Palace, or Villa Dubrovnik for the two of you, but it depends on what is more important to you - beach, pool, proximity to town, seclusion, spa, etc.