I think www.octopustravel.com is a great website for locating hotels. I\'ve dealt with them several times…they are very reliable and provide competitive prices.

You must be careful when choosing a hotel in Istanbul as some areas can be a bit dodgy. I would recommend hotels around the Taksim area. Here are some good (and reasonable) suggestions:

Richmond Hotel ****

http://www.richmondhotels.com.tr/eng/istanbulrichmond.asp

Elite Hotel ****

http://www.elitehotel.com.tr/

Pera Palas (built in 1892)

http://www.perapalas.com/en/index.php

Madison Hotel ****

You can also check the Istanbul City Guide for tips on where to stay/eat, places to see, cultural events etc.  

http://english.istanbul.com/?Vst=2