You have three options: Turkcell, Avea, and Vodafone.
If you bring your "unlocked" 900Mhz/1800Mhz GSM cell phone with you, then you can buy a prepaid card and start using any of these cell phone carriers' services. You have to register your cell phone with the government. The customer service at the carriers' sales office will help you with this without a charge. If you don't register, then your phone will be blocked from making or receiving calls just in a month or so. You can only register one phone each year. So, you can't register and use two different cell phones if you bring them from abroad.
To be honest, the mobile phone company websites don't really have a lot of information on the prices of prepaid cards --in English language. I would suggest going to one of the city centers like Taksim, Beyoglu or Kadikoy and find their stores. Or, you can go to one of the shopping malls that has all of these carriers' stores in them. Then, you can easily find an English speaking customer representative and find out about their prices.
Istanbul Cevahir in Sisli has all of them.
If you are going to be in Istanbul all the time, then I would choose the one that is the cheapest (calling numbers on the same carrier, and the others). Please note that 50% of the cell phone users are on Turkcell. If you are going to be traveling all around the country -off the beaten path-, then I would either do a research with interactive maps/asking around, or stick with the biggest player in the game.
If you have a smart phone, than please note that 3G is available at all of these carriers, however, you will be able to find wi-fi almost everywhere you go.
