1. What are differences between Cyclone II and Cyclone III And Spartan III related to speed and compatibility?
The speed is about the same. I know quite well the Cyclone serie, between the generation speed gets slightly better and power consumption (and price) decreases.
Not enough LEs, I would recommend at least 10KLEs, 16KLEs is better and 25KLEs is perfect.
3. Can i use some other boards based on some other FPGA.
Look at the terasic ones : DE0, DE1 or DE2 or I you prefer Xilinx, the latest ATLYS board from digilent looks awesome.
4. Can it be done to connect SDRAM controller to TG68.
I have a design running with an SDRAM controller, a TG68 (soon to be replaced by an AO68000) and the Minimig core.
It takes ~13KLEs.