Your most welcome Almas.
Between December and February, the temperature might be as high as 12-13 Celcius degrees to as low as -2 Celcius degrees. However, in Istanbul it usually sticks around 7-10 degrees by morning and one or two points lower by night. It's all surrounded by sea, so don't worry it won't bee too cold in december in Istanbul.
However, Ankara and surroundings can be as low as 2 degrees by ay and Cappadocia region (the land where Göreme, Ürgüp, Fairy Chimneys lay) is slightly colder than Ankara. It also depends a lot on chance, such as we had a very mild (around 15 by day and 10 by night) december last year (2007) however, it was -2 to -4 by february and a lot of snow. So, I assume, any month besides february is great for weather here, with a little luck, you can even get some sunshine as well. Not sun-till-your-bones-get-dried, but some sunshine that'll get you keep moving. 
Turkey is worth a visit. Here, we've a lot of intresting places as well as great deal of history. You'll see with your own eyes how differet culture and cities are as you move west from east and probably suprised, as most of my friends are, with Istanbul and it'll even exceed what you are thinking about it.
I work very close to Cappadocia region (like half-an-hor's drive westwards) so I'll be happy to answer your questions about anywhere in Turkey, with as much as I can. I've been around this country for 15 years.