I have a Leadtek A6600GT TDH (the AGP model).
Stock clock speeds are 500MHz for GPU (under 3D), 900MHz for memory.
I currently have it overclocked to 550MHz / 1100 MHz.
I went as high as 570 / 1175, but in some (not all) 3D applications I was getting a few artifacts, so I went a little lower.
For overclocking i used Coolbits.
My card has a fan. The temperatures are 40-45C idle, 80-85C under load (and overclocked). However, the temperatures from when it wasn't overclocked were around those same values. In the driver, it says that the threshold temperature is 127C, so I don't worry about it.
Have fun with it, and take it slow.