Also,
Here are more tests:

4x 512 mb apache server, 1 nginx server( 2048 mb) :700 requests per second
4x 2048 mb apache server, 1 nginx server( 2048 mb) :700 requests per second

In conjuction with the data giving before, it shows that nginx is the bottle neck. But the cpu and memory usage are very very low. I am using "top" and sars to check activity.

Also, using and activity on apache seems to be very low as well.