I've been there in July, August and September. It is HOT. I love Istanbul, it is a city with so many layers that you can really only peel of a few during the first visit.
If I were to go in the month of July again, I would make absolutely sure that I was in an air conditioned hotel. Two reasons -- if you have to open the windows at night, you will get mosquito attacks and it can be loud (Turks like music at night). If you have to close the windows, you will axfixiate.
The Bosphorus is incredible, and you can do all sorts of tours by boat. In the neighborhood of Bacasir (sp), under the Sultan Bridge, the fish restaurants are lined up for the tourists. I prefer to go over to the Asian side and drive up the coast toward the Black Sea -- the fish restaurants are cheaper and better, IMO.
While it is hot out, visit the traditional tourist spots: Topkapi (you have to pay extra to see the Harem, but I think it worth it), the Blue Mosque, the Hajia Sophia, and the Bazaar. In the late afternoon, hang out a local cafe and in the evening, hit the local street bazaars in all of the downtown tourist areas. I have bought lapis, silver, and linens there.
Watch your diets. Turkish food is WONDERFUL, but there is a lot of oil...it is sometimes hard to tell if the oil has been reused or not. I have ended up in the clinic in Istanbul with food poisoning. The private clinics are very good, because there are wealthy Turks and above all Germans which support them.
I would allow four or five days for a first tour to get oriented. Then go back when it is cooler....
cheers