Download and run GPU-z. It's a very useful small program. On the "sensors" tab, you can read the GPU temperature and fan speed.
http://www.techpowerup.com/downloads/2627/techpowerup-gpu-z-v0-8-7/
If temperature is above 80 C it's too high. You will need a case with better airflow or just give your case, GPU and power supply a thorough but careful cleaning. Google how to do it if you're not sure. You can use damp cloth, brush or an air compressor/vacuum cleaner.