I am using Autobench to test the nginx server performance. The numbers i generated above are when the response times are still very low. After than, more requests per second starts to error out.

IE:
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

the range of testing was from 500 -1000 requests per second. the response times and network IO increased steadily until it hit 700, then response times increase dramatically and network IO stayed flat at that point.