The application CPU-Z will tell you good basic ram timings for your ram. Some ram may need 'loose' timings.

On to your problem. When you populate all 4 ram slots the memory controller is really taxed. Your ram voltage may need to be bumped up as high as 2.2V to get the machine to post. Raise the voltage slowly (starting from 1.8V) until it posts then run memtest from cd to check if the timings/voltages are good. Rinse and repeat untill memtest is good then boot up and double check the reported timings in cpu-z. Now run some prime95 stress tests to verify everything is stable.