setup nginx for higher load

hi all,

I have a centos 6 machine on netmagic cloud with specs 8 gb ram 4 core. now i av installed nginx on it n configured it with all possible optimization as provided on http://blog.martinfjordvald.com/2011/04/optimizing-nginx-for-high-traffic-loads/ still my nginx can establish 300 connection at max n can serve only 150-200 requests per second.can ne one suggest nething apart from given on above link. or do i need to purchase a higher configuration machine.