In my opinion, you're going to the most fantastic urban photography site in the world. Not only that, you'll probably like the Turks. The food is good, and the city is pretty safe.
Gotta go to: The Kapali Carsi, the Grand Bazaar or Covered Market, it's called all those things. One of the greatest in the region. Look for old cameras. You won't believe the place, you probably won't want to leave. Gotta see the Haghia Sophia. Also, gotta go over to Asia, and shoot back, especially in the late afternoon, when the sun turns gold and the city gets a golden haze over it: with the low, broad domes, Istanbul looks like something from a Sci-Fi novel. Great people shooting on the Ataturk Bridge over the Golden Horn. Actually, great people shooting just about everywhere. The Topaki Palace is really...palace-like. You know, in the Eastern Sultan sense of the word...
Equipment... I'd take your 1DsII, at least one lens that will go wide, one that will go long, and maybe something for portraits. (But be sure to ask about close shots of people; the Turks are more relaxed about Islam than in most places in the Middle East, but some religious Muslims don't like having their image taken.) I shoot Nikons so I'm not familiar with the Canon system, but do take a solid wide angle lens. In some of the narrow crooked streets, you can't get back; and you may want to ask about shooting inside some of the stores. I loved the little musical instrument district, narrow streets, strange instruments.Take at least a monopod, or preferably a tripod, if you can handle the weight. Be about as careful with your gear as you would be, say, in Greenwich Village. There's some marvelous stuff behind glass, another place a short, and fast, lens would be good. Also, for dimmer areas, of which there are many, a flash is necessary; if you could manage 2, with a slave, that would be even better.
Dress conservatively. If you really plan to get into it, I'd take a pair of grey or khaki slacks, a light-weight dark sport coat, and a shirt with a collar. Depending on where you plan to go, a tie would not be a bad idea. Doesn't have to be expensive, just neat. That will get you into all religious sites -- a lot of religious sites, Christian as well as Muslim, think American casual dress, especially shorts, is disrespectful. I think the No. 1 most-hassled tourists are the guys in shorts and photo vests, or quasi-military style stuff with epaulets and bush hats. If you're wearing shorts and a photo vest, or a bush hat, you ain't from there. If you're wearing a blue coat and grey slacks and dark shoes, you might be. The city you're going to is one of the most sophisticated in the world, the equivalent of Paris. Safari gear is not needed.
You're gonna have a great time. Unless, of course, you're killed and eaten.